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Palaeoceanography relies on the assumption that parameters measureable in sediment cores correlate reliably to hydrographic parameters. Areas of dynamic watermass mixing, which generally have steep hydrographic gradients, therefore provide both a tempting target (large spatial and temporal differences in temperature and salinity) and a significant challenge (high flow velocities and high “noise”) for palaeoceanographers. Here, we investigate the ability of parameters measured on core tops to be used as the basis for simulating regional hydrography within one globally important area of watermass mixing, the Gulf of Cadiz. Using grainsize, sediment composition, benthic foraminiferal assemblage and benthic foraminiferal stable isotope analysis it is possible to build an effective framework for qualitative constraint of the position of the modern Mediterranean Outflow Water (MW) Plume. We propose that the Gulf of Cadiz slope can be sub-divided into 8 hydrographic zones reflecting these parameters; Proximal MW, Core MW1 (steep slopes), Core MW1 (gentle slopes), Distal MW1, Distal MW2, Lower Limit of MW plume, North Atlantic Deep Water and Ambient Atlantic Water. We anticipate that compilation of time-slice data will reveal hydrographic sub-divisions of the slope for the past in a similar manner, improving our understanding of past changes in the size and position of the MW plume, but more work needs to be done before a secure protocol for quantitative reconstruction can be created. Stable isotope analysis alone is insufficient for the task, with δ13C behaving in a non-conservative manner and δ18Ocalcite acting in an ambiguous way due to the competing influences of δ18Owater and temperature. It is likely that if a single parameter that unambiguously determines the difference between Mediterranean Water and Atlantic Water can be identified, combination with δ18Ocalcite will allow extension of our qualitative analysis into a quantitative means of palaeohydrographic reconstruction for this region. Similar complexity to that found in this study would be expected in any mixing region, and in particular on the majority of sediment drifts. We recommend that core top surveys similar to this study are performed on these sediment drift systems before any form of quantitative palaeohydrology is attempted.

A novel technique was applied to estimate differences in core shortening in three gravity cores taken at the same core location on the Portuguese continental slope using different coring devices. No obvious deformational features are visible in the fresh core sediments; the isotope stratigraphy and abundance maxima of ice-rafted debris, representing North Atlantic Heinrich-events, indicate identical stratigraphic range in all three cores. However, one of the cores is significantly shorter than the others. X-ray radiographs of the cores reveal that the shorter core shows typical deformation structures, whereas the two other cores are lacking signs of deformation. This serious disturbance had likely gone unnoticed had it not been for the X-ray radiographs. As an approach to semi-quantitatively estimate the core shortening, we used the fragmentation and displacement of the pyritized trace fossil Trichichnus that is easily recognizable in X-ray radiographs through its high contrast. The Trichichnus data indicate that a shortening of 50–60% occurred in the lower part of the shorter core. This estimate is in good agreement with variations in apparent sedimentation rates for the interval considered. Accurate flux rates are essential for our understanding marine biogeochemical cycles in general and the marine budgets of nutrients such as carbon and phosphorus in particular. X-ray radiographs are very useful in assessing the intactness of the sedimentary records and the presented method has potential to become a valuable tool in correcting sedimentation rates in disturbed gravity cores.

Biogenic structures in Late Quaternary sediments from the southwestern Iberian continental slope were studied by using X-ray images from two cores from 580 and 1750 m water depth. Eight different ichnocoenoses were observed: indistinct bioturbation, Planolites-dominated, Thalassinoides-dominated, Chondrites-dominated, Planolites and Thalassinoides-dominated, pyritized microburrows such as Trichichnus and ‘Mycellia’-dominated, Chondrites, Trichichnus, and ‘Mycellia’-dominated and Zoophycos. Variations of the ichnocoenoses within the cores show a striking correlation with climatically induced changes in the hydrographical regime, i.e. current strength and bathymetric position of the Mediterranean Outflow Water (MOW). Comparison of the response of the ichnocoenoses to changes in bottom-water conditions and substrate between the two cores studied indicate that bottom-water oxygenation and enrichment of particulate organic matter at the base of the MOW layer are the primary factors controlling the ichnocoenoses. The traces even recorded short-term climatic changes such as the Younger Dryas cold event. The spatial and temporal distribution patterns are in good agreement with earlier models of the MOW history, which gives reason to see a refinement of trace fossils as a complementary tool for paleoceanographical studies.

High-resolution palaeoclimate records recovered from the Iberian margin in core MD95-2040 exhibit large fluctuations in oceanographic conditions over the last 190 ka. Large-scale cooling of the surface ocean is indicated by the presence of the polar planktonic foraminifer Neogloboquadrina pachyderma (sinistral), and in some instances the occurrence of ice-rafted debris (IRD). Ice-rafting episodes were prevalent in both of the last two glacials with greater intensity in Stages 2 through 4, than in Stage 6. The six youngest Heinrich events are well defined during the last glacial but detrital carbonate is absent from Heinrich layers HL6, HL5 and HL3. Dansgaard–Oeschger stadial-equivalent sub-millennial IRD deposition events have been detected, in particular during Stage 3, allowing a good match with the cooling displayed in the Greenland ice core (GISP2). Sea-surface temperature off Portugal in Stage 6 was in general warmer than during the last glacial, pointing towards a weaker southward influence of polar water masses. Ice rafting occurred mainly in mid-MIS (Marine Isotope Stage) 6 (between 173 and 153 kyr) as a group of poorly differentiated, short-duration quasi-continuous events, mainly marked by the high abundance of sinistral N. pachyderma. Differences exist in IRD composition relative to the last glacial, with a reduced Canadian-derived detrital carbonate component, combined with an important contribution of volcanic particles. The lower magnitude and higher frequency of these events suggest that the higher temperatures would have induced iceberg waning closer to the source areas.

The distribution of trace fossils and benthic foraminiferal assemblages in two sediment cores from 2160 m water depth at the northern and 1100 m at the southern Portuguese continental margin was used to reconstruct the bottom-water oxygenation during the last 40 ka. Emphasis was given to the Heinrich events, during which enhanced meltwater flux associated with the iceberg surges reduced or even halted deep water formation in the North Atlantic. Condensed trace-fossil tiering structures and high proportions of low-oxygen-tolerant benthic foraminifera suggest a major drawdown of bottom-water oxygenation during Heinrich events. Using the benthic foraminiferal oxygenation index (BFOI) of Kaiho (1991, 1994) that links benthic foraminiferal communities to ambient bottom-water oxygenation, we extracted apparent oxygen concentrations from benthic assemblages. BFOI values suggest low-oxic conditions during Heinrich events 1 and 4 with mean oxygen levels of 2.6 to 2.8 ml l −1 and high-oxic conditions with 4.1 ml l −1 during Heinrich event 2 at the shallower site. High proportions of low-oxygen-tolerant foraminiferal species and BFOI minima are recorded a few centimetres below the maximum concentrations of ice-rafted debris, indicating that the original microhabitat depth of dysoxic species is preserved beneath the Heinrich layers even though bioturbation continued during the events. Trace-fossil tiering calibrated with the redox boundary in low-oxic environments and dysoxic ichnotaxa commonly associated with black shales indicate oxygen levels lower than those implied by the BFOI. Oxygenation estimates derived from trace-fossil tiering denote low-oxic conditions during H1 (13.3 ka) with oxygen concentrations slightly above 1 ml l −1 , and dysoxic conditions during H2 (20.6 ka), H3 (27 ka) and H4 (33.5 ka) with oxygen concentrations between 0.1 and 1 ml l −1 . During the Younger Dryas, bottom waters likely were low oxic. Negative excursions of benthic carbon isotope values provide independent control on the oxygen drawdown during Heinrich events. Considering a bottom-water oxygenation of 6.1 to 6.5 ml l −1 during the last glacial, the benthic δ 13 C minima reveal an oxygen depletion to values between 3.4 and 5.0 ml l −1 during H1 through H4 at the shallower core site. Differences between estimates from benthic δ 13 C, BFOI and trace-fossil tiering suggest that benthic foraminiferal communities are rather robust to oxygen depletion if the level does not fall below 3 ml l −1 , whereas burrowing organisms react sensitively to depth variations of the redox boundary in near-surface sediments due to even small changes of bottom-water oxygenation.