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Abstract: In many palaeoceanographic studies low carbonate contents in Neogene deep-sea sediments are interpreted as dissolution events related to prominent shifts in intermediate and deep-water circulation. In the middle to late Miocene such carbonate crash events were a widespread phenomenon throughout the world's ocean basins. However, low carbonate contents may also derive from terrigenous dilution and carbonate production changes in surface waters as shown for late Miocene records of the South Atlantic (Diester-Haass et al., 2004; Westerhold et al., 2005). In order to contribute to solve this question we investigated the youngest of the Miocene crash events (10.5Ma to 9.0Ma) at a prominent key location of global deep water circulation, i.e. the Equatorial Atlantic Ceará Rise depth transect (ODP Sites 926, 927 and 928). By analysing different carbonate preservation proxies like carbonate content, silt grain size parameters, sand content, fragmentation index and mass accumulation rates we were able to quantify dissolution, dilution and production, and to assess previous conflicting interpretations. Critical evaluation of the different dissolution proxies and comparison to former studies revealed that silt preservation proxies do not entirely reproduce the depth dependant dissolution in the Miocene sediments, as indicated by foraminifer fragmentation. In the investigated late Miocene time slice three minima of carbonate accumulation were detected. The predominant control on these events changed from dissolution of carbonate to lowered production. The last event at 9.6Ma is accompanied by an increased preservation but reduced carbonate accumulation. During this time the first prominent pulses of Northern Component Water (NCW) occurred, sourced most probably from the Labrador Sea. This may be understood as a first short term precursor of long time persistent and much stronger NCW production that is characteristic for younger periods and marks the turnover to the modern type of basin-to-basin fractionation between the Atlantic and Pacific. Furthermore, preservation trends of Caribbean and Ceará Rise records were in phase in contrast to former hypotheses that suggested antithetical preservation during carbonate crash events. The similarity of the preservation records in the studied ocean basins in conjunction with the inconsistency in timing of the CC-events and proposed NCW estimates indicates that changes in the carbon cycle other than dissolution controlled the CC events prior to ~9.9Ma. [Copyright &y& Elsevier]

Abstract: The submarine Dakar Canyon incises the continental margin at the transition of the hyperarid Sahara to the semiarid Sahel Zone, and acts as an effective pathway for gravity-driven sediment transport. Four gravity cores recovered directly from the canyon axis were investigated in order to reconstruct the sedimentation processes in the Dakar Canyon during the Late Quaternary. In addition, a hemipelagic record from the northern levee of the canyon was analysed for monitoring background sediment supply, which is dominated by dust input in the area. Coarse terrigenous silt size data and high Ti/Ca ratios reflect overall increased higher dust supply during the last two peak glacials. During these times wide-extensive sand sea covered the exposed shelf almost completely. However, in interglacial periods wind stress diminished considerably and only minor amounts of dust were supplied to the outer shelf and continental slope. Two major periods of turbidite depositions are recorded in intervals from final glacial sea level lowstands to early deglacial sea level rise of the last two glacial/interglacial cycles (i.e. between 141 and 131kyr BP and from 23.2 to 14.2kyr BP). These turbidite deposits consist of sandy to silty sediments. Detailed grain size analyses were used to reconstruct the sedimentary characteristics and flow processes of these turbidity currents. A much higher frequency in turbidite activity occur around 135kyr BP in contrast to the second interval around 18kyr BP, suggesting a higher sediment budget in the source area. Based on the sedimentological investigation of the turbidites we provide a schematic model for the sedimentation processes in the Dakar Canyon. [Copyright &y& Elsevier]

Abstract: A major outcome from studying the sedimentary archives of the Timiris Canyon off Mauritania is that climate shifts in the Saharan hinterland not only influenced offshore pelagic sedimentation processes, but also controlled turbidite activity in the canyon system. Sediment supply to the slope, predominantly by eolian dust and remobilised eolian sands, was greatest during hyperarid glacial and deglacial periods and, due to overall increased Trade wind strengths, upwelling induced pelagic productivity attained highest magnitudes. Turbidite activity in the Timiris Canyon responded rapidly to these shifts in the surrounding sedimentary regime on the continental slope. During high-glacial and early deglacial times, the tributary gullies on the uppermost slope were rapidly and repetitively filled, subsequently turbidity currents were released and large turbulent flows passed through the lower Timiris Canyon, leading to overspill sequences on its levees. In addition to this scenario, partial flooding of the exposed outer shelf during successive sea-level rises in MIS 3 may have caused re-mobilization of huge aeolian dune fields, that had expanded close to the shelf edge during glacial exposure. During the Holocene, no turbidite activity is recorded in the Lower Timiris Canyon, whereas a core from a widened area in the Upper Timiris Canyon reveals an interesting climate controlled 900-year cyclicity of carbonate- and mud-turbidite events throughout the Holocene. [ABSTRACT FROM AUTHOR]

Sediments from the ODP Site 1085A were studied to investigate the impacts of global cooling in the Middle and Late Miocene on the climate in Southwestern Africa. The size composition of the sediment was analysed emphasising the silt fraction. A comparison with the modern grain size distribution and suitable transport processes made it possible to assign specific transport processes to the grain size composition. Three processes are considered for transport of terrigeneous silt: while there was no evidence found for (1) transport by ocean currents, the analyses showed signals of (2) wind transport indicating dry conditions associated with a cool climate and (3) fluvial transport that points to humid and warm conditions. Three climatic phases were defined. The first phase from 13.8 to 11.8 Myr reveals a stable humid climate in Southwest Africa independent of the Antarctic glaciations. During the second phase from 11.8 to 10.4 Myr the regional climate cooled considerably but was not drier. Additionally, the climate during this phase reacted to the Antarctic glaciations. This cooling-trend continued during phase 3 from 10.4 to 9.0 Myr with a significant increase in dust input, pointing to overall drier conditions. However, fluvial transport still remained as the main source. [ABSTRACT FROM AUTHOR]

Abstract: Sediment records from continental shelves and slopes might provide paleoenvironmental information in the highest temporal resolution but are often hampered due to strong erosional and reworking processes. Here, we present a Holocene sedimentary record from an exceptional shelf mud belt depocenter off northern Mauritania, compared to a second sediment core located inside a large canyon system at the adjacent continental slope. Both records are of outstandingly continuous and highest temporal resolution (9a/cm) and are investigated by sedimentological and geochemical methods. A series of sharply defined, recurrent dust peaks is preserved in the shelf archive. Each event has lasted for a single decade only and seems to coincide with an individual turbidite bed in the canyon. A joint mechanism should, thus, be responsible for both of these deposits and we suggest a regional atmospheric trigger. Only short-lasting Trade wind strengthening would cause such pronounced aridity over western Saharan Africa. The effect would be massive dust export to shelf and slope. Recently developed high resolution aridity and humidity records from western Africa support the existence of these events over the Holocene and identify them as being controlled by the Atlantic system as far south as 19°N. [Copyright &y& Elsevier]

Abstract: Large-scale mass wasting is an important sedimentary process along the northwest African margin, and is related to high sediment accumulation rates under an ocean margin upwelling regime. Although the margin is generally arid with limited fluvial input, additional sediment supply comes from wind-borne Saharan dust. Recent mapping of the margin off Mauritania has revealed a major sediment slide, here called the Mauritania Slide Complex, as it comprises elements of true sliding as well as more mobile distal debris flow. Seismic data image stacked slide deposits separated by undisturbed stratified sediments indicating that undisturbed sediment accumulation was interrupted by several phases of slope failure. A series of stepped headwalls, 25–100m high, represents the source area of the youngest slide event, which most likely occurred as retrogressive type of failure. The area of seafloor affected by this mass movement is ∼30,000km2, while the deposit volume is ∼600km3. The uppermost debrite unit, which has been 14C dated at 10.5–10.9ka, forms a broad tongue extending down to the lower slope. This debrite comprises a vertical succession of three different layers of matrix types, with a predominantly outer shelf source at the base and pelagite-dominated composition at the top. The complete sequence of three layers was deposited at a mid slope position, whereas only the upper layers reached the lower slope. A thick pile of sediments with outer shelf/upper slope derived biogenic and terrigenous debris-rich sediments at the base and hemipelagic sediments on top failed at an upper to mid slope location and disintegrated into a layered debris flow on its down-slope journey. [Copyright &y& Elsevier]

The current issue of global warming and the role of the ocean in global exchange of CO2 increases the interest in solid budgets of marine carbonate production and dissolution. The present study utilizes grain-size composition of pelagic sediments in order to trace spatial and temporal variability of carbonate sedimentation in the South Atlantic for the Holocene and Last Glacial Maximum (LGM, 19–23 cal kyr BP). A decrease in grain size (e.g. sand content, mean grain size of coarse carbonate silt) indicates increased carbonate dissolution as a result of increased fragmentation of calcareous microfossils. The spatial grain-size pattern suggests a threshold water depth below which a gradual grain-size decrease becomes increasingly rapid. This water depth is considered as the sedimentary lysocline. For the Holocene time slice, a constant, gradual decrease of foraminifer carbonate of about 5–10% per 1000 m water depth above the lysocline gives evidence for supra-lysoclinal dissolution. The water depth of the lysocline for the Holocene is tied to the interface of North Atlantic Deep Water and Antarctic Bottom Water (AABW) ( ca 4100 m). Submarine ridges which restrict intrusion of AABW into the Angola Basin cause an asymmetry in carbonate preservation across the Mid-Atlantic Ridge. The lysocline was reconstructed at ca 3100 m for the LGM. These data suggest that the ca 1000 m rise of the lysocline eradicated the Holocene east–west asymmetry. [ABSTRACT FROM AUTHOR]

Abstract: A record of variations in winnowing and dissolution was established for Miocene (19–7 Ma) deep-sea sediments at Site 1265 from the Walvis Ridge, South Atlantic. In this study we apply a new preservation index based on the abundance of coarse calcareous silt fraction (10–63 μm). The Miocene record of this silt preservation index corresponds well with the conventional foraminiferal preservation index and variations in sand content. Preservation ranging from good to moderate during the Miocene indicates that Site 1265 is located above and close to the foraminiferal lysocline. Grain size, mass accumulation rates of carbonate, and organic carbon records indicate that changes in productivity and carbonate production play a minor role in variations of calcium carbonate concentration. The Miocene variation in carbonate content is caused mostly by a combination of winnowing and dissolution. Significant decreases of preservation at 17.3, 16.3, 13.8, 13.0, 11.6, and 10.4 Ma coincide with Miocene glacial events (Mi-events), suggesting that dissolution was caused by an increase of Southern Component Water (SCW) influence, which occurred as a response to the intensification of Antarctic ice sheet development. The SCW acts as the major deep-water source during the global Miocene cooling. At 10.4 Ma a change to overall better preservation points to a weakening of SCW that occurred as a response to the initiation or strengthening of North Atlantic Deep Water (NADW). [Copyright &y& Elsevier]

Abstract: Enhanced Atlantic overturning during the Pliocene was first proposed almost 10 yrs ago. Evidence for this Pliocene super conveyor scenario has been collected using a number of proxies (e.g., benthic δ13C, Nd isotopic composition of manganese crusts). The present study contributes to the existing evidences by using carbonate dissolution and current vigour history of early Pliocene sediments from the Ceará Rise (ODP Sites 927 and 929). In order to reveal carbonate dissolution history, a number of commonly used and newly established proxies were applied, i.e., sand and carbonate contents, foraminifer fragmentation index, Bulloides Dissolution Index and carbonate silt grain-size distributions. Terrigenous silt grain-size distributions were used to unravel variations in relative current strength and sediment input to the two sites. Overall good carbonate preservation at the shallow Site 927 (3314 m water depth) shows that this level was bathed in North Atlantic Deep Water throughout the early Pliocene. The contrastingly poor carbonate preservation record of the deeper Site 929 (4358 m water depth, at present exposed to Antarctic Bottom Water) is frequently interrupted by phases of good carbonate preservation. These results indicate that the depth of the calcite lysocline was mainly tied to present level (∼4200 m water depth), and sometimes even dropped to water depths greater than 4360 m due to even more enhanced circulation. Surprisingly the expansion of NADW is not clearly reflected by an increase in current speed as shown by continuously fine terrigenous grain size. [Copyright &y& Elsevier]

The Atlantic is regarded as a huge carbonate depocenter due to an on average deep calcite lysocline. However, calculations and models that attribute the calcite lysocline to the critical undersaturation depth (hydrographic or chemical lysocline) and not to the depth at which significant calcium carbonate dissolution is observed (sedimentary calcite lysocline) strongly overestimate the preservation potential of calcareous deep-sea sediments. Significant calcium carbonate dissolution is expected to begin firstly below 5000 m in the deep Guinea and Angola Basin and below 4400 m in the Cape Basin. Our study that is based on different calcium carbonate dissolution stages of the planktic foraminifera Globigerina bulloides clearly shows that it starts between 400 and 1600 m shallower depending on the different hydrographic settings of the South Atlantic Ocean. In particular, coastal areas are severely affected by increased supply of organic matter and the resultant production of metabolic CO2 which seems to create microenvironments favorable for dissolution of calcite well above the hydrographic lysocline. [Copyright &y& Elsevier]

Carbonate preservation records from a number of drill sites in the North Atlantic and adjacent Norwegian–Greenland Sea (NGS) are used to reconstruct variations in North Atlantic Deep Water (NADW) production over the past 3 Myr. Before the initiation of major Northern Hemisphere glaciation, good carbonate preservation was recorded in the North Atlantic and the NGS supporting the superconveyor model of accelerated NADW formation in the late Pliocene. After the inception of main Northern Hemisphere glaciation, NADW formation in the NGS was blocked-off during the period 2.8–1.9 Ma. Carbonate was only badly preserved due to (1) low production of carbonate shells in surface waters, (2) sluggish renewal of deep waters induced by a rather stable sea-ice cover, and/or (3) production of carbonate-corrosive dense brines during sea-ice formation (e.g. sea ice dissolution mode). In contrast, contemporaneous good preservation in the adjacent North Atlantic indicates efficient NADW production. During the middle Matuyama (1.9–1.4 Ma), the first intrusions of the Proto-Norwegian Current into a narrow corridor in the southeastern NGS are evidenced by much better carbonate preservation. A decrease both in carbonate content and preservation towards the north and west of this corridor indicates the proximity of the polar front and gives evidence that NADW production was efficiently triggered by the Atlantic water entrainment mode. During the past 1.2 Myr carbonate preservation patterns in the NGS clearly reflect major global events like the Mid-Pleistocene Transition and the mid-Brunhes dissolution event. The onset of the Mid-Pleistocene Transition at 1.2 Ma is characterized by a complete shift to higher carbonate contents in the southeastern NGS and Labrador Sea. Overall, good preservation during both interglacials and glacials is only interrupted by high-frequency, short-term dissolution spikes, which were induced by ice sheet collapse and development of extensive meltwater lids. As a consequence, NADW was produced nearly continuously during glacials in the Nordic Seas. However, due to its lower density it was entrained into intermediate water levels in the North Atlantic and, thus, enforced the intermediate water circulation loop, whereas a decrease in lower-NADW production is observed contemporaneously. [Copyright &y& Elsevier]

In this study, we test various parameters in deep-sea sediments (bulk sediment parameters and changes in microfossil abundances and preservation character) which are generally accepted as indicators of calcium carbonate dissolution. We investigate sediment material from station GeoB 1710-3 in the northern Cape Basin (eastern South Atlantic), 280 km away from the Namibian coast, well outside today’s coastal upwelling. As northern Benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr (Volbers et al., submitted), GeoB 1710-3 sediments reflect these changes in upwelling productivity. Results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity. Accordingly, these conventional proxy parameters misrepresent, to some extent, the extent of calcium carbonate dissolution. These results were verified by an independent dissolution proxy, the Globigerina bulloides dissolution index (BDX′) (Volbers and Henrich, submitted). The BDX′ is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr, with the exception of one pronounced dissolution event at early oxygen isotopic stage (OIS) 6.The early OIS 6 is characterized by calcium carbonate contents, sand contents, and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr. At the same time, the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased, as are the rain ratio, the fragmentation index, and the BDX′. The sedimentary calcite lysocline rose above the core position and GeoB 1710-3 sediments were heavily altered, as attested to by the unusual accumulation of pellets, aggregates, sponge spicules, radiolaria, benthic foraminifera, and planktonic foraminiferal assemblages.Solely the early OIS 6 dissolution event altered the coarse fraction intensely, and is therefore reflected by all conventional calcium carbonate preservation proxies and the BDX′. We attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes: (a) a prominent change in the deep-water mass distribution within the South Atlantic and (b) intense degradation of organic material within the sediment (preserved as maximum total organic carbon content) creating microenvironments favorable for calcium carbonate dissolution. [Copyright &y& Elsevier]

Abstract We present X-Ray Fluorescence (XRF) Scanner measurements from a 6 m long sediment core (GeoB12624-1) on the upper slope of Tanzania to reconstruct the climatic evolution in East Africa since the Last Glacial Maximum (LGM). Log-ratios of Fe/Ca and Ti/Ca are indicative for sediment discharge of the Rufiji River, which is controlled by climatic conditions in the Rufiji catchment area. The most significant changes in major elemental composition occurred at 15.1 and 7.4 ka highlighted by the regime shift index values. The data set records distinct precipitation peaks during the early Holocene. This corresponds a maximum in the Northern Hemisphere (NH) summer insolation and results in a transition from the arid LGM to the humid early Holocene. Our geochemical record also indicates that this climatic transition was interrupted by two severe droughts that occurred during NH cold intervals: the Heinrich stadial 1 (HS1) and the Younger Dryas (YD). Through a comparison with other nearby paleoclimatic records, we suggest that arid climatic conditions only occurred in East Africa north of 8–10°S, whereas in southern East Africa around 15–20°S increased humidity during the HS1 and YD prevailed. We thus conclude that these two drought events were caused by a southward migration of the Intertropical Convergence Zone (ITCZ) which was fostered by the NH cooling during the HS1 and YD. Hence, our new geochemical record clearly documents that the East African climate not only responded to low-latitude insolation forcing on sub-orbital time scales, but also, was strongly influenced by high-latitude cooling during the HS1 and YD periods. Highlights • High-resolution climatic record from the Tanzanian upper slop since the LGM. • Climatic transition in East Africa following the summer insolation since the LGM. • The YD and HS1 drought throughout East Africa responding to shift of the ITCZ. • SSTs in the Indian Ocean playing a minor role in controlling East African climate. [ABSTRACT FROM AUTHOR]

We present a high-resolution geochemical and grain-size record from a Holocene sediment core off the Pangani River mouth, Tanzania. Elemental ratios between biogenic elements and Al (i.e., Ca/Al, Mg/Al, and Sr/Al) are mainly influenced by terrigenous dilution on carbonate concentration and/or limitation of carbonate production as a result of variations in the supply of fine-grained terrigenous sediments of the Pangani River. Such elemental ratios increased significantly at the end of the mid-Holocene between 5 and 3.5 ka, demonstrating a gradual transition from the humid early and mid-Holocene to the arid late Holocene in East Africa. Among the elemental ratios between terrigenous elements and Al, Si/Al and K/Al ratios correlate to grain-size variation, indicating a change in sedimentation regime. Fe/Al and Ti/Al ratios show that the sediment source area has shifted from the terrestrial volcanic region of Tanzania (Fe, Ti rich) to the coastal and inner-shelf regions (Fe, Ti poor) around 7.5 ka, in response to arid climate and high sea level. Our geochemical results correspond with a sea-surface temperature record derived from the same sediment core, indicating that the end of the East African Humid Period could have been gradual and related to the cooling water in the western Indian Ocean. [ABSTRACT FROM AUTHOR]

Turbidity current is one of the most important mechanisms for rapid transportation of terrigenous sediments from the continental margin to the deep sea. Although extensive work on turbidite systems has been carried out globally, the Tanzanian margin, off East Africa, is poorly understood. This paper will therefore use a well-dated high-resolution marine core (GeoB12624-1), obtained during RV Meteor Cruise M75/2, from the upper Tanzanian continental slope, offshore the Rufiji River delta, to demonstrate which environmental parameters, e.g., climate versus sea level, control the temporal distribution of the turbidite deposits during the last deglaciation and the Holocene. Results show that the turbidite deposits are composed of coarser-grained sediments with normally graded bedding. The thickness, mean grain size and frequency of the turbidite beds were estimated to detect turbidity current activity since the Last Glacial Maximum (LGM). A total of 12 turbidite beds (TI-T12) was recognized and divided into three intervals, based on the presence and intensity of the turbidite deposits. (I) a glacial sea-level lowstand (LGM and Heinrich Stadial 1; 19.3–14.6 ka): suppressed turbidity current activity due to arid conditions in the hinterland and sea-level lowstand. (II) A deglacial sea-level rise period (Bølling-Allerød interval to the early Holocene; 14.6–8 ka): turbidity current activity started to be strengthened during the Bølling-Allerød warming interval (T1), followed by a first increase in turbidite frequency, thickness and mean grain-size during the Younger Dryas (T2–T4) and a second increase during the early Holocene (T5–T10). (III) An interglacial sea-level highstand period (mid-to late Holocene; 8–2.5 ka): turbidity current activity diminished from the mid- (T11, T12) to late Holocene. This temporal distribution of turbidite deposits indicates that turbidity currents were most active during the last sea-level rise, synchronous with more humid climate in the hinterland, than during a sea-level lowstand (e.g., LGM) when climate was more arid. Thus, we propose that it is the climate conditions in the hinterland that form the primary controlling factor for turbidity current activity of Tanzanian slope. The remobilization of lowstand sediments on the Tanzanian continental margin during a sea-level rise, combined with shedding sediments towards the study area from the main canyon/channel, also plays a role in supplying new sediments to the turbidite system. These new data demonstrate that, in specific continental margin, high fluvial discharge will promote sediment transfer along the shelf and slope even during sea-level rise and highstand. [ABSTRACT FROM AUTHOR]

Abstract: Based on a well-established stratigraphic framework and 47 AMS-14C dated sediment cores, the distribution of facies types on the NW Iberian margin is analysed in response to the last deglacial sea-level rise, thus providing a case study on the sedimentary evolution of a high-energy, low-accumulation shelf system. Altogether, four main types of sedimentary facies are defined. (1) A gravel-dominated facies occurs mostly as time-transgressive ravinement beds, which initially developed as shoreface and storm deposits in shallow waters on the outer shelf during the last sea-level lowstand; (2) A widespread, time-transgressive mixed siliceous/biogenic-carbonaceous sand facies indicates areas of moderate hydrodynamic regimes, high contribution of reworked shelf material, and fluvial supply to the shelf; (3) A glaucony-containing sand facies in a stationary position on the outer shelf formed mostly during the last-glacial sea-level rise by reworking of older deposits as well as authigenic mineral formation; and (4) A mud facies is mostly restricted to confined Holocene fine-grained depocentres, which are located in mid-shelf position. The observed spatial and temporal distribution of these facies types on the high-energy, low-accumulation NW Iberian shelf was essentially controlled by the local interplay of sediment supply, shelf morphology, and strength of the hydrodynamic system. These patterns are in contrast to high-accumulation systems where extensive sediment supply is the dominant factor on the facies distribution. This study emphasises the importance of large-scale erosion and material recycling on the sedimentary buildup during the deglacial drowning of the shelf. The presence of a homogenous and up to 15-m thick transgressive cover above a lag horizon contradicts the common assumption of sparse and laterally confined sediment accumulation on high-energy shelf systems during deglacial sea-level rise. In contrast to this extensive sand cover, laterally very confined and maximal 4-m thin mud depocentres developed during the Holocene sea-level highstand. This restricted formation of fine-grained depocentres was related to the combination of: (1) frequently occurring high-energy hydrodynamic conditions; (2) low overall terrigenous input by the adjacent rivers; and (3) the large distance of the Galicia Mud Belt to its main sediment supplier. [Copyright &y& Elsevier]

Abstract: During Leg 195 of the Ocean Drilling Program, Site 1202 was drilled in the subtropical northwestern Pacific Ocean beneath the Kuroshio (“Black Current“) between northern Taiwan and the Ryukyu Island Arc on the northern flank of the I-Lan Ridge at 1274 m water depth. The upper 110 m of the Site 1202 section, composed of dark grey calcareous silty clay, provide an expanded record of environmental changes during the last 28 kyr. The sediments were deposited at high sedimentation rates between 3.0 and 5.0 m/kyr and peak values of 9.0 m/kyr between 15.1 and 11.2 ka BP. Variations in the modes and sources of detrital sediment input, as inferred from sediment granulometry, mineralogy, and elemental XRF-scanner data, reflect changes in environmental boundary conditions related to sea-level changes, Kuroshio variability, and the climate-driven modes of fluvial runoff. The provenance data point to increased sediment supply from northwestern Taiwan between 28 and 19.5 ka BP and from East China sources between 19.5 and 11.2 ka BP. The change in provenance at 19.5 ka BP reflects increased fluvial runoff from the Yangtze River and strong sediment reworking from the East China Sea shelf in the course of increased humidity and postglacial sea-level rise, particularly after 15.1 ka BP. The Holocene was dominated by sediments that originated from rivers in northeastern Taiwan. For the pre-Holocene period prior to 11.2 ka BP, low portions of sortable silt (63–10 μm) show that the Kuroshio did not enter the Okinawa Trough, because of low sea-level. In turn, high proportions of sortable silt and sediment provenance from northeastern Taiwan point to strong ocean circulation under the direct and persistent influence of the Kuroshio during the Holocene. The reentrance of the Kuroshio to the Okinawa Trough was heralded by two pulses in relative current strengthening at 11.2 and 9.5 ka BP, as documented by stepwise increases in sortable silt in the lower Holocene section. From a global perspective, environmental changes in the southern Okinawa Trough show affinities to climate change in the western Pacific warm pool with little influence of climate teleconnections from the North Atlantic realm, otherwise seen in many other marine and terrestrial palaeoclimate records from southeastern Asia. [Copyright &y& Elsevier]

Abstract: Variations in deposition of terrigenous fine sediments and their grain-size distributions from a high-resolution marine sediment record offshore northwest Africa (30°51.0′N; 10°16.1′W) document climate changes on the African continent during the Holocene. End-member grain-size distributions of the terrigenous silt fraction, which are related to fluvial and aeolian dust transport, indicate millennial-scale variability in the dominant transport processes at the investigation site off northwest Africa as well as recurring periods of dry conditions in northwest Africa during the Holocene. The terrigenous record from the subtropical North Atlantic reflects generally humid conditions before the Younger Dryas, during the early to mid-Holocene, as well as after 1.3 kyr BP. By contrast, continental runoff was reduced and arid conditions were prevalent at the beginning of the Younger Dryas and during the mid- and late Holocene. A comparison with high- and low-latitude Holocene climate records reveals a strong link between northwest African climate and Northern Hemisphere atmospheric circulation throughout the Holocene. Due to its proximal position, close to an ephemeral river system draining the Atlas Mountains as well as the adjacent Saharan desert, this detailed marine sediment record, which has a temporal resolution between 15 and 120 years, is ideally suited to enhance our understanding of ocean–continent–atmosphere interactions in African climates and the hydrological cycle of northern Africa after the last deglaciation. [Copyright &y& Elsevier]

Abstract: This study deals with the variability of biogenic carbonate and terrigenous depositional patterns in the Cape Basin area of the South Atlantic during the middle to the late Miocene focusing on ODP Sites 1085 and 1087 sediments at the Benguela Upwelling area, Southwest Africa. The investigations are based on grain-size analysis and clay mineralogy. The sedimentary records at both sites provide information about climate and environmental changes. The direct input of terrigenous material from southwestern Africa is driven by a complex interaction of river and wind transport, and ocean currents. The considerable amount of dust input derived from the Namib Desert started to become important after 11.2 Ma, pointing to an increased supply of cool intermediate waters to mid-latitude surface waters in the upwelling regions, which coincided with the establishment of the wind-driven Benguela upwelling system. A major drop in CaCO3 concentration between 10.4 and 10.1 Ma is caused mainly by changes in calcareous nannoplankton production, while another drop between 9.6 and 9 Ma is triggered by a combination of production changes of calcareous nannoplankton and dilution, in response to high shelf supply during global lowering of the sea level. [Copyright &y& Elsevier]

Abstract: In this study, the coccolith compositions of 213 surface sediment samples from the South Atlantic and Southern Ocean were analysed with respect to the environmental parameters of the overlying surface waters. From this data set, the abundance patterns of the main species and their ecological affinities were ascertained. In general, Emiliania huxleyi is the most abundant species of the recent coccolith assemblages in the study region. However, the lower photic zone taxa, composed of Florisphaera profunda and Gladiolithus flabellatus often dominate the assemblages between 20°N and 30°S. If E. huxleyi is excluded, Calcidiscus leptoporus and F. profunda become the most abundant species, each dominating discrete oceanographic regimes. While F. profunda is very abundant in the sediments underneath warmer, stratified surface waters with a deep nutricline, Calcidiscus leptoporus is encountered in high-productivity environments. Furthermore, the results of a canonical correspondence analysis reveal affinities of Gephyrocapsa spp., Helicosphaera spp. and Coccolithus pelagicus for intermediate to higher nutrient conditions in a well-mixed upper water column. In contrast, Gladiolithus flabellatus seems to be associated with high temperatures and salinities under low-nutrient conditions. Based on the relative abundances of Calcidiscus leptoporus, F. profunda, Gladiolithus flabellatus, Helicosphaera spp., Umbilicosphaera foliosa, Umbilicosphaera sibogae and a group of subordinate subtropical species, six surface sediment assemblages have been identified, which reflect the distribution and characteristics of the overlying surface waters. Their distribution appears to be mainly a function of the relative position of the nutricline and thermocline in the overlying photic zone. [Copyright &y& Elsevier]

Abstract: Middle to late Miocene calcareous nannofossil data of ODP Site 1085 from the eastern South Atlantic off Namibia were analysed to document spatial and temporal changes in surface-ocean circulation, upwelling initiation, and associated productivity. Our data show that calcareous nannofossils constitute a significant part of the carbonate fraction throughout the investigated interval from 12.5 to 7.7 million years (Ma). Highest numbers of calcareous nannofossils (up to 38,000×106 nannofossils g−1 sediment) were observed during the intervals 9.9 to 9.7 and 8.7 to 8.0 Ma. These elevated numbers of calcareous nannofossils may generally be linked to the initiation of upwelling at about 10 Ma in the studied region. In contrast, diminished numbers of calcareous nannoplankton, as in the interval 9.6 to 9.0 Ma, probably characterise time intervals of weaker productivity resulting in a decrease of nannofossil carbonate contents in the sediments of Site 1085. This decrease in nannofossil production could be one possible explanation for the major CaCO3 depression in between 9.6 and 9.0 Ma. Coccoliths of the genus Reticulofenestra are the most abundant taxa. Their occurrences are characterised by changes in the investigated time interval. In addition, Coccolithus pelagicus, Calcidiscus leptoporus and Umbilicosphaera spp. contribute a common part of the assemblage. Calcareous nannofossils account for more than half of the carbonate, with peak contribution up to 80% at 8.8 Ma. [Copyright &y& Elsevier]

The terrigenous fraction of seabed sediments recovered along the north-west African continental margin illustrates spatial variability in grain size attributed to different transport mechanisms. Three subpopulations are determined from the grain-size analyses (n = 78) of the carbonate-free silt fraction applying an end-member modelling algorithm (G. J. Weltje, 1997). The two coarsest end-members are interpreted as representing aeolian dust, and the fine-grained end-member is related to fluvial supply. The end-member model thus allows aeolian fallout to be distinguished from fluvial-sourced mud in this area. The relative contributions of the end-members show distinct regional variations that can be related to different transport processes and pathways. Understanding present-day sediment dispersal and mixing is important for a better understanding of older sedimentary records and palaeoclimate reconstructions in the region. [ABSTRACT FROM AUTHOR]

Antarctic Intermediate Water (AAIW) is an essential component of the Atlantic meridional overturning circulation (AMOC) contributing to balance the southward flow of North Atlantic Deep Water (NADW). However, the role of AAIW in Holocene abrupt climate changes remains poorly understood. Here we reconstruct changes in the flow strength of AAIW based on a high temporal resolution paleocurrent record from the Southwest Atlantic. Superimposed on a slight increase in AAIW strength at ~ 7 ka BP, a succession of millennial-scale AAIW variations is recognized in our paleocurrent records indicating a highly variable intermediate water circulation throughout the Holocene. Although variations in the strength and position of the Southern Westerlies Winds (SWW) are proposed to greatly influence the formation and circulation of AAIW, we cannot confirm such a potential SWW-AAIW linkage since our records of AAIW flow strength do not correlate to Holocene shifts of the SWW across the Atlantic. However, our data shows a good correspondence with abrupt variations in the AMOC with enhanced (reduced) northward advection of AAIW during periods of reduced (enhanced) NADW circulation. These results provide evidence for a Holocene AAIW-NADW see-saw. Thus, although the exact forcing mechanism remains unresolved, we suggest that Holocene perturbations in AAIW exerted a significant impact on the AMOC. [ABSTRACT FROM AUTHOR]

Equatorial East Africa experienced significant variations in paleoclimatic and paleoceanographic conditions during the Holocene. These environmental changes influenced sedimentation patterns on the continental shelf. To date, however, little is known about the sediment source, its transport to, and deposition on, the Tanzanian shelf. This paper presents a new high-resolution Holocene sedimentary record off northeast Tanzania (equatorial East Africa) and provides insights into how sedimentation patterns responded to climatic and oceanographic changes during the Holocene. Based on grain-size distribution patterns and mineral assemblages, three types of shelf sediments were identified: Type I (fine-grained terrigenous sediment) is dominated by clay minerals that originated from continental weathering; Type II (coarse-grained terrigenous sediment) is mainly composed of feldspar and quartz, derived from reworking of pre-existing deposits; and Type III (biogenic marine sediment), with low- and high-magnesium calcite, was produced by marine carbonate-secreting organisms. The high input of Type I sediment during the early Holocene (10–8 cal kyr BP) was caused by river mouth bypassing. This supply-dominated regime was controlled by intense river discharge and subsequent resuspension of mud in shelf settings, responding to the humid climate in the hinterland and sea-level rise with low rate off Tanzania. The first occurrence of Type II sediments was around 8 cal kyr BP and dominated when sedimentation rates lowered. This accommodation-dominated regime was caused by shoreface bypassing due to an arid climate and sea-level highstand. Type III sediments increased significantly from the early to late Holocene, resulting from the weakening dilution effect of the terrigenous component. The sedimentation pattern on the Tanzanian shelf shifted from allochthonous to autochthonous sedimentation constrained by climatic changes and relative sea-level fluctuations at the end of the early Holocene. Our results also suggest that, with respect to sea-level rise, high sedimentation rate on the continental shelf could be caused by high terrigenous sediment input as a result of a dynamic interplay between the shelf topography and humid climatic conditions inland. [ABSTRACT FROM AUTHOR]

Abstract: Fine-grained sediment depocenters on continental shelves are of increased scientific interest since they record environmental changes sensitively. A north–south elongated mud depocenter extends along the Senegalese coast in mid-shelf position. Shallow-acoustic profiling was carried out to determine extent, geometry and internal structures of this sedimentary body. In addition, four sediment cores were retrieved with the main aim to identify how paleoclimatic signals and coastal changes have controlled the formation of this mud depocenter. A general paleoclimatic pattern in terms of fluvial input appears to be recorded in this depositional archive. Intervals characterized by high terrigenous input, high sedimentation rates and fine grain sizes occur roughly contemporaneously in all cores and are interpreted as corresponding to intensified river discharge related to more humid conditions in the hinterland. From 2750 to 1900 and from 1000 to 700cal a BP, wetter conditions are recorded off Senegal, an observation which is in accordance with other records from NW-Africa. Nevertheless, the three employed proxies (sedimentation rate, grain size and elemental distribution) do not always display consistent inter-core patterns. Major differences between the individual core records are attributed to sediment remobilization which was linked to local hydrographic variations as well as reorganizations of the coastal system. The Senegal mud belt is a layered inhomogeneous sedimentary body deposited on an irregular erosive surface. Early Holocene deceleration in the rate of the sea-level rise could have enabled initial mud deposition on the shelf. These favorable conditions for mud deposition occur coevally with a humid period over NW-Africa, thus, high river discharge. Sedimentation started preferentially in the northern areas of the mud belt. During mid-Holocene, a marine incursion led to the formation of an embayment. Afterwards, sedimentation in the north was interrupted in association with a remarkable southward shift in the location of the active depocenter as it is reflected by the sedimentary architecture and confirmed by radiocarbon dates. These sub-recent shifts in depocenters location are caused by migrations of the Senegal River mouth. During late Holocene times, the weakening of river discharge allowed the longshore currents to build up a chain of beach barriers which have forced the river mouth to shift southwards. [Copyright &y& Elsevier]

Abstract: The relationship of sea-level changes and short-term climatic changes with turbidite deposition is poorly documented, although the mechanisms of gravity-driven sediment transport in submarine canyons during sea-level changes have been reported from many regions. This study focuses on the activity of the Dakar Canyon off southern Senegal in response to major glacial/interglacial sea-level shifts and variability in the NW-African continental climate. The sedimentary record from the canyon allows us to determine the timing of turbidite events and, on the basis of XRF-scanning element data, we have identified the climate signal at a sub-millennial time scale from the surrounding hemipelagic sediments. Over the late Quaternary the highest frequency in turbidite activity in the Dakar Canyon is confined to major climatic terminations when remobilisation of sediments from the shelf was triggered by the eustatic sea-level rise. However, episodic turbidite events coincide with the timing of Heinrich events in the North Atlantic. During these times continental climate has changed rapidly, with evidence for higher dust supply over NW Africa which has fed turbidity currents. Increased aridity and enhanced wind strength in the southern Saharan–Sahelian zone may have provided a source for this dust. [Copyright &y& Elsevier]

Abstract: To improve our understanding of the Late Pleistocene and Holocene carbonate system of the western Arabian Sea a high-resolution sedimentary record off Somalia has been analysed. The 15.26-m-long piston core 905 comprises a complete record of the last 90,000 years. We have measured concentrations of carbonate minerals, i.e., aragonite, calcite, Mg-calcite, and element ratios (Sr/Ca) together with pteropod counts and an estimation of the preservation state of pteropod shells to trace temporal changes in carbonate production and preservation. The Sr/Ca ratio shows strong similarities to the aragonite percentage and the δ18O record of the planktic foraminifer Neogloboquadrina dutertrei. High Sr/Ca ratios together with fragments of corals found in the coarse fraction indicate that most of the aragonite is of shallow water origin (high-Sr aragonite) and pteropods contribute much less than expected. High resuspensional input of shallow-marine aragonite occurs during sea-level highstands (interglacials) and low input during lowstands (glacials). The Mg-calcite concentration record resembles the whole pteropod abundance and pteropod shell preservation records confirming the use of Mg-calcite in combination with pteropod preservation proxies to reconstruct past fluctuations in carbonate dissolution. Preservation of aragonite and Mg-calcite increases during stadials, H-equivalents, YD and late Marine Isotope Stage (MIS) 3. During late MIS 5/early MIS 4 and in the Late Holocene absence of few pteropods as well as low Mg-calcite weight percentages point to strong dissolution of aragonite and Mg-calcite. [Copyright &y& Elsevier]

Seafloor mapping of the Mauritania margin by multibeam bathymetry as well as shallow and high-resolution seismics in combination with core data has revealed a wide variety of seafloor features, which are primarily generated by turbidity currents and landslides. The largest observed features in the study area are two submarine slides, which both have affected over 30,000 km2 of seafloor. The Cap Blanc Slide is not considered to be a significant geohazard at the present day as it has a minimum age of 165 ka and shows no evidence of recent reactivation. In contrast, the complex Mauritania Slide Complex is characterized by a retrogressive failure style and has an age of only 10.5-10.9 cal. ka B.P.We found no indication for recent reactivation of this slide, but more data are needed to assess the present seafloor stability in this area. Turbidity current activity on the margin is evidenced by numerous gullies, canyons and channels. The largest of these is the >450 km-long meandering Cap Timiris Canyon.During the present Holocene sea-level highstand, however, terrigenous sediment supply to the modern outer shelf has been restricted and most of the conduits are therefore thought to be largely inactive, at least in terms of large sand-rich flows. [ABSTRACT FROM AUTHOR]

Abstract: In order to better understand Late Quaternary pelagic aragonite preservation in the western Arabian Sea we have investigated a high-resolution sediment core 905 off Somalia. Pteropod preservation is enhanced in times of reduced monsoon-driven productivity, indicated by low amounts of Corg and low barium to aluminium (Ba/Al) ratios. All periods corresponding to Heinrich events in the North Atlantic are represented by maxima in shell preservation of the common pteropod Limacina inflata (LDX values<2, except for H5-equivalent with a poorer shell preservation, LDX>2.66). Good shell preservation is also found during stadials at 52.1–53.2, 36, 33.2, and 31.9 ka. Relative abundance of pteropods and their fragments in the coarse fraction reaches maxima during Marine Isotope Stage (MIS) 5.2, during time-equivalents of Heinrich events 4–6 and in stadials at ∼53, ∼42.5, and 41.4 ka. On longer time scales, the pteropod abundance corresponds to the ‘Indo-Pacific carbonate preservation type’ with poor preservation during interglacials and better preservation during glacials. Late MIS 5 to early MIS 4 sections (84.1–64.8 ka) and the Late Holocene interval (6.5–0 ka) of core 905 contain only traces of pteropods. The early Holocene (9.2–6.5 ka) part is characterized by low pteropod amounts. Between 64.8 and 43.4 ka strong fluctuations occur and an intermediate average relative pteropod abundance is revealed. Between 43.4 and 9.2 ka the highest amounts in relative pteropod abundance in core 905 are observed. Besides the regional monsoonal influence on deepwater chemistry, changes in deepwater circulation occurring on glacial/interglacial and stadial/interstadial time scales might have affected pteropod preservation. However, it remains elusive whether 1) deep water formation in the Arabian Sea, 2) inflow of Glacial North Atlantic Intermediate Water or 3) change in water mass properties of the Circumpolar Deep Water (which is the water mass currently bathing this site) contributed to the observed pteropod preservation pattern. [Copyright &y& Elsevier]

Examines the seasonal export production and surface accumulation rates of coccolithophore and carbonate sedimentation in Canary Islands. Increase in coccolith sedimentation in February and March; Preference of Gephyrocapsa species for higher productivity conditions; Effect of the dissolution processes on the deep sediments.

This study intends to review and assess the middle to late Miocene Carbonate Crash (CC) events in the low to mid latitudes of the Pacific, Indian, Caribbean and Atlantic Oceans as part of the global paleoceanographic reorganisations between 12 and 9 Ma with an emphasis on record preservation and their relation to mass accumulation rates (MAR). In the Eastern Pacific the accumulation changes in carbonate and opal probably reflect an El-Niño-like state of low productivity, which marks the beginning of the CC-event (11.5 Ma), followed by decreased preservation and influx of corrosive bottom waters (10.3 to 10.1 Ma). At the same time in the Atlantic, carbonate preservation considerably increases, suggesting basin-to-basin fractionation. The low-latitude Indian Ocean, the Pacific and the Caribbean are all characterised by a similar timing of preservation increase starting at ~9.6–9.4 Ma, while their MARs show drastic changes with different timing of events. The Atlantic preservation pattern shows an increase as early as 11.5 Ma and becomes even better after 10.1 Ma. The shallow Indian Ocean (Mascarene plateau) is characterised by low carbonate accumulation throughout and increasing preservation after 9.4 Ma. At the same time, the preservation in the Atlantic, including the Caribbean, is increasing due to enhanced North Atlantic deep-water formation, leading to the increase in carbonate accumulation at 10 Ma. Moreover, the shoaling of the Central American Isthmus might have helped to enhance Caribbean preservation after 9.4 Ma. Lower nannoplankton productivity in the Atlantic should have additionally contributed to low mass accumulation rates during the late CC-interval. Overall, it can be inferred that these carbonate minima events during the Miocene may be the result of decreased surface ocean productivity and oceanographically driven increased seafloor dissolution. [ABSTRACT FROM AUTHOR]

The Mar del Plata (MdP) Canyon at the Argentine continental margin is incorporated into a major contourite depositional system, built by the incursion of southern-sourced water masses affecting the seafloor at different waters depths. The new sedimentological, morphological and hydro acoustic data provide novel insights into contour and turbidity current interactions in mid-slope (blind) canyons, which do not have a connection to the shelf or an onshore river system. Such canyons are capable to record climate-related ocean stratification changes, current variability, and slope stability. Three sediment cores were obtained along the MdP Canyon thalweg covering the last 20,000 years and compiled with two cores from the adjacent Ewing Terrace. Turbidity-current activity within the MdP Canyon was limited to the time interval from Last Glacial Maximum (LGM) to the late deglacial. During the LGM and early deglacial, turbidites reached both the proximal sector and the distal northern flank of the canyon. During the late deglacial only the proximal sector was characterized by turbidite deposition. Similarities in mineralogy and grain-size data indicate that the material transported by the turbidity currents originated from the mid-slope Ewing Terrace. Glacial turbidity-current activity was most probably favored by increased sediment supply along the Ewing Terrace from a shallowed and/or enhanced glacial Antarctic Intermediate Water (AAIW) nepheloid layer. These sediments were trapped by the MdP Canyon, in particular at the head area. During the late deglacial, a displacement or limited AAIW nepheloid layer resulted in less sediment transfer along the Ewing Terrace and immediate accumulation in the MdP Canyon head restricting turbidite deposition to the proximal sector of the canyon. In general, contourite-turbidite interactions provide valuable information on variations in thermohaline circulation such as AAIW distribution and current strength. • Thermohaline circulation induced turbidity-current activity. • Contour and turbidity current interaction restricted to late last glacial period. • Intermediate nepheloid layer sediment supply induces sediment instabilities. • Transformation of sediment flows into low-density turbidity currents. [ABSTRACT FROM AUTHOR]