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Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum. [ABSTRACT FROM AUTHOR]

Abstract: The sensitivity to temperature of Mg/Ca ratios in the shallow-infaunal benthic foraminifera Uvigerina spp. has been assessed. Core-top calibrations over ∼1–20°C show a range in sensitivity of 0.065–0.084mmol/mol/°C but few data are available spanning the temperature range anticipated in deep-sea records over glacial–interglacial cycles. In contrast to epibenthic foraminiferal species, carbonate ion saturation appears not to affect Mg/Ca significantly. A method based on estimating the ratio of the temperature sensitivity of foraminiferal Mg/Ca to that of δ18Ocalcite shows that sensitivity for Mg/Ca at the high end of the observed core-top range (∼0.1mmol/mol/°C) is required for consistency with LGM–Holocene differences in each property as constrained by independent proxy data. This is supported by a Mg/Ca record for Uvigerina spp. generated for the Southern Ocean over the past 440,000 years from Ocean Drilling Program Site 1123 (Chatham Rise, New Zealand). The record shows variability that correlates with climate oscillations. The LGM deep ocean temperature derived from the Mg/Ca record is −1.1±0.3°C. Transformation to temperature allows estimates to be made of changes in bottom water temperature and seawater δ18O and comparison made with literature records. Analysis reveals a ∼2.5-kyr lead in the record of temperature over calcite δ18O and a longer lead over seawater δ18O. This is a reflection of larger phase offsets at eccentricity periods; phase offsets at tilt and precession are within error zero. [Copyright &y& Elsevier]

Abstract: Benthic foraminiferal magnesium/calcium ratios were determined on one hundred and forty core-top samples from the Atlantic Ocean, the Norwegian Sea, the Indian Ocean, the Arabian Sea and the Pacific Ocean, mostly at sites with bottom water temperatures below 5 °C. Mg/Ca ratios are consistently lower, by ∼0.2 mmol/mol, in samples cleaned using oxidative and reductive steps than using oxidative cleaning. Differences between Cibicidoides species have been identified: Mg/Ca of Cibicidoides robertsonianus > Cibicidoides kullenbergi > Cibicidoides wuellerstorfi. Comparison with bottom water temperatures support observations of lowered Mg/Ca of C. wuellerstorfi at temperature below ∼3 °C compared with values predicted by published calibrations and from other Cibicidoides species. Hydrographic data shows that carbonate ion saturation (Δ[CO3 2−]) decreases rapidly below this temperature. An empirical sensitivity of Δ[CO3 2−] on Mg/Ca has been established for C. wuellerstorfi of 0.0086±0.0006 mmol/mol/μmol/kg. A novel application using modern temperatures and Last Glacial Maximum temperatures derived via pore fluid modelling supports a carbonate ion saturation state effect on Mg incorporation. This may significantly affect calculated δ 18Oseawater obtained from foraminiferal δ 18O and Mg/Ca temperature. [Copyright &y& Elsevier]

A Cenozoic multi-species record of benthic foraminiferal calcite Sr/Ca has been produced and is corrected for inter-specific offsets (typically less than 0.3 mmol/mol) and for the linear relationship between decreasing benthic foraminiferal Sr/Ca and increasing water depth. The water depth correction, determined from Holocene, Late Glacial Maximum and Eocene paleowater-depth transects, is ∼0.1 mmol/mol/km. The corrected Cenozoic benthic foraminiferal Sr/Ca record ranges from 1.2 to 2.0 mmol/mol, and has been interpreted in terms of long-term changes in seawater Sr/Ca, enabling issues related to higher-resolution variability in Sr/Ca to be ignored. We estimate that seawater Sr/Ca was ∼1.5 times modern values in the late Cretaceous, but declined rapidly into the Paleogene. Following a minimum in the Eocene, seawater Sr/Ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late Miocene. By assuming scenarios for changing seawater calcium concentration, and using published carbonate accumulation rate data combined with suitable values for Sr partition coefficients into carbonates, the seawater Sr/Ca record is used to estimate global average river Sr fluxes. These fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity/tectonic outgassing to calculate changes in global average river 87Sr/86Sr through the Cenozoic. The absolute magnitude of Sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties. Nevertheless, our results suggest that river Sr flux increased from 35 Ma to the present day (roughly two-fold) accompanied by an overall increase in 87Sr/86Sr (by ∼0 to 0.001). Between 75 and 35 Ma, river 87Sr/86Sr also increased (by ∼0.001 to 0.002) but was accompanied by a decrease (two- to three-fold) in river Sr flux. [Copyright &y& Elsevier]

Cd/Ca and δ13C were measured on bottom and pore water samples, and samples comprising dead individuals of six species of benthic foraminifera, including Cibicidoides wuellerstorfi, Uvigerina peregrina and Melonis barleeanum, from throughout the sediment mixed layer at three well-characterised sites in the Northeastern Atlantic. ‘Living’ (i.e., Rose Bengal stained) U. peregrina and M. barleeanum from one of the three sites were also analysed. Co-existing living and dead foraminifera of the same species from the same site have similar Cd/Ca and δ13C, and show no significant down core variability. Therefore, comparison of δ13C in foraminifera with bottom water and pore waters was used to estimate average calcification depths within the sediment for each species and thereby determine DCd based on the Cd concentrations at these depths. Pore waters are 2–4 times more enriched in Cd than bottom waters; consequently, DCd values are different from estimates based on bottom water Cd. Results give DCd of ∼1 for all the infaunal species, with no significant water depth dependence. DCd for C. wuellerstorfi based on bottom water Cd are 3.2±1.1 at 3600 m water depth and 3.9±1.3 at 1900 m water depth, being consistent with DCd estimated from culture experiments. The results suggest that the depth dependence of DCd based on bottom water Cd may be partly explained by a pore water influence on the test chemistry for infaunal species. [Copyright &y& Elsevier]

Demonstrates that the variation of magnesium and calcium ratios with temperature is similar for eight species of planktonic foraminifera. Methods, which include using a multi-species record from the Last Glacial Maximum in the North Atlantic Ocean; Proof that reconstructed surface water oxygen isotope data from all foraminiferal species record the same glacial-interglacial change; Potential for probing past ocean-climate interactions.

Reports on the investigation of the Southern Ocean's nutrient utilization in surface water to determine the relationship between the atmospheric carbon dioxide concentration and the oceanic carbon uptake, using the principles of paleoceanography. Concentration of cadmium and phosphorus that is explained by chemical fractionation; Reconstruction of surface water phosphate concentrations; Results that include information on phosphate utilization during the Last Glacial Maximum.

Presents information on a study which produced a deep-sea temperature record for the past 50 million years from the magnesium/calcium ratio (Mg/Ca) in benthic foraminiferal calcite. Change in Earth's climate; Timing and magnitude of global climate change during the Cenozoic period; Basis for the Mg/Ca paleothermometry; Global ice volumes.

Geophysical estimates of mid-ocean ridge axial heat fluxes (2–4 &times 10[sup 12] W) and of the total hydrothermal flux (9 ± 2 × 10[sup 12] W) are well established. Problems arise in calculation of water fluxes because of uncertainties in (a) values of off-axis fluxes and (b) the partition of axial heat flow between high-temperature black smoker and lower-temperature diffuse flow. Of the various geochemical methods of estimating fluxes,[sup 3] He/heat data are extremely variable, the Mg method is sensitive to flank fluxes, Sr isotopes agree with geophysical estimates only if flank fluxes are important, Li isotopes data are consistent with geophysical values, and Ge/Si ratios give low fluxes, which may reflect low-temperature processes not yet fully quantified. Estimates of hydrothermal heat and water fluxes derived from these approaches are presented as are hydrothermal chemical fluxes at the ridge axis, off axis, and as affected by hydrothermal plumes. [ABSTRACT FROM AUTHOR]

Sediments within and immediately above pillow lavas of the Troodos Massif of Cyprus show abnormally high concentrations of iron, manganese, arsenic and copper. Most likely they contain precipitates related to volcanism along a Tethyan rise. In addition, silica has been leached from lavas and has partly remobilized to form chert layers above the ferromanganese sediments. As a result of hydrothermal activity some horizons are enriched in copper, lead, zinc and nickel, and manganese and arsenic have been mobilized in the sediment column. [ABSTRACT FROM AUTHOR]

The data given in the literature for the infra-red identification of carbonate minerals is summarized, and a technique is described which may be used to identify magnesite, smithsonite, dolomite, rhodochrosite, calcite, aragonite, strontianite, cerussite and witherite by means of their infra-red absorption bands. Two quantitative applications of the use of infra-red spectroscopy are considered and techniques are outlined for the determination of calcite-dolomite ratios in carbonate rocks, and calcite-aragonie ratios in shell material. [ABSTRACT FROM AUTHOR]

This Discussion Meeting Issue of the Philosophical Transactions A had its genesis in a Discussion Meeting of the Royal Society which took place on 10-11 October 2011. The Discussion Meeting, entitled 'Warm climates of the past: a lesson for the future?', brought together 16 eminent international speakers from the field of palaeoclimate, and was attended by over 280 scientists and members of the public. Many of the speakers have contributed to the papers compiled in this Discussion Meeting Issue. The papers summarize the talks at the meeting, and present further or related work. This Discussion Meeting Issue asks to what extent information gleaned from the study of past climates can aid our understanding of future climate change. Climate change is currently an issue at the forefront of environmental science, and also has important sociological and political implications. Most future predictions are carried out by complex numerical models; however, these models cannot be rigorously tested for scenarios outside of the modern, without making use of past climate data. Furthermore, past climate data can inform our understanding of how the Earth system operates, and can provide important contextual information related to environmental change. All past time periods can be useful in this context; here, we focus on past climates that were warmer than the modern climate, as these are likely to be the most similar to the future. This introductory paper is not meant as a comprehensive overview of all work in this field. Instead, it gives an introduction to the important issues therein, using the papers in this Discussion Meeting Issue, and other works from all the Discussion Meeting speakers, as exemplars of the various ways in which past climates can inform projections of future climate. Furthermore, we present new work that uses a palaeo constraint to quantitatively inform projections of future equilibrium ice sheet change. [ABSTRACT FROM AUTHOR]

Abstract: Salinity increase in the subtropical gyre system may have pre-conditioned the North Atlantic Ocean for a rapid return to stronger overturning circulation and high-latitude warming following meltwater events during the Last Glacial period. Here we investigate the Gulf Stream – subtropical gyre system properties over Dansgaard–Oeschger (DO) cycles 14 to 12, including Heinrich ice-rafting event 5. During the Holocene and Last Glacial Maximum a positive gradient in surface dwelling planktonic foraminifera δ18O (Globigerinoides ruber) can be observed between the Gulf Stream and subtropical gyre, due to decreasing temperature, increasing salinity, and a change from summer to year-round occurrence of G. ruber. We assess whether this gradient was a common feature during stadial-interstadial climate oscillations of Marine Isotope Stage 3, by comparing existing G. ruber δ18O from ODP Site 1060 (subtropical gyre location) and new data from ODP Site 1056 (Gulf Stream location) between 54 and 46 ka. Our results suggest that this gradient was largely absent during the period studied. During the major warm DO interstadials 14 and 12 we infer a more zonal and wider Gulf Stream, influencing both ODP Sites 1056 and 1060. A Gulf Stream presence during these major interstadials is also suggested by the large vertical δ18O gradient between shallow dwelling planktonic foraminifera species, especially G. ruber, and the deep dwelling species Globorotalia inflata at site 1056, which we associate with strong summer stratification and Gulf Stream presence. A major reduction in this vertical δ18O gradient from 51 ka until the end of Heinrich event 5 at 48.5 ka suggests site 1056 was situated within the subtropical gyre in this mainly cold period, from which we infer a migration of the Gulf Stream to a position nearer to the continental shelf, indicative of a narrower Gulf Stream with possibly reduced transport. [Copyright &y& Elsevier]

Abstract: We report here the results of a multi-proxy study to reconstruct surface water pCO2 concentrations in the northern Arabian Sea. Our results show that δ 11B and Mg/Ca measurements of the planktonic foraminifer Globigerinoides sacculifer yield consistent pCO2 values with those reconstructed from the δ 13C of alkenones when used in conjunction with foraminifera δ 13C and Cd/Ca values. They reveal that this area of the oceans has been a constant source of CO2 to the atmosphere during the interval 5–29ka, and that the intensity of this source was greatest between 11 and 17ka, when atmospheric CO2 levels were rising rapidly. We interpret our data as reflecting variation in the strength of the Asian Summer Monsoon (ASM), thus indicating that the strength of the ASM varied in phase with summer insolation over the Tibetan plateau between 5 and 29ka. In contrast to a previous study (), we observe no significant lag between the rise in insolation and the response of the ASM. Rather, our data support a recent study by that northern hemisphere climatic forcing factors play a greater role in controlling the intensity of the ASM during times of intense monsoon activity, and that the southern hemisphere forcing is more important during times of weak monsoons. [Copyright &y& Elsevier]

Abstract: We have measured the magnesium (Mg) isotope ratios in 45 rivers including 16 of the largest rivers in the world, covering a range of geologic, tectonic and climatic drainage basin environments. The range in riverine δ26Mg is 2.5‰, half the variation in terrestrial rock. Although the Mg isotopic composition of the source rock is important for small rivers, at a global scale lithology is of limited significance for Mg isotope ratios in rivers. Part of the variability at a global scale may be attributed to fractionation in the weathering environment. The rivers analysed constitute 30% of the global Mg riverine flux to the oceans and a flux weighted Mg isotope composition of global runoff has been estimated at −1.09‰. Even taking into account uncertainty, this is distinct from seawater at −0.82‰. This difference arises either from the fractionation of Mg isotope ratios in the ocean and/or a Mg budget which is not at steady state. The difference is consistent with fractionation by carbonate precipitation. In the simplest steady state scenario, where the oceanic mass balance is maintained by riverine input and hydrothermal and dolomite output, Mg isotope ratios imply a minimum dolomite Mg flux of 9% of the total output Mg flux. The implications of the distinct isotopic composition of the riverine flux from modern seawater, is that the Mg isotope ratio of seawater has probably varied outside of current analytical uncertainty. Mg isotope ratios may therefore provide a valuable new tracer in palaeo-oceanography. [Copyright &y& Elsevier]

Abstract: Published studies show that ice rafted debris (IRD) deposition preceding Heinrich (H) events H1 and H2 in the NE Atlantic was derived from the NW European ice sheets (NWEIS), possibly offering clues about ice sheet sensitivity and stability, and the mechanisms that caused periodic collapse of the Laurentide Ice Sheet (LIS). We present detailed lithological and geochemical records, including radiogenic isotope fingerprinting, of IRD deposits from core MD01-2461, proximal to the last glacial British Ice Sheet (BIS), demonstrating persistent instability of the BIS, with significant destabilisation occurring 1.5–1.9 kyr prior to both H1 and H2, dated at 16.9 and 24.1 kyr BP, respectively, in the NE Atlantic. Paired Mg/Ca and δ 18O data from the surface dwelling Globigerina bulloides and subsurface dwelling Neogloboquadrina pachyderma sinistral are used to determine late-glacial variability of temperature, salinity and stratification of the upper water column. A picture emerges that the BIS was in a continuing state of readjustment and never fully reached steady state. Increased sea surface temperatures appear to have triggered the episode of NWEIS instability preceding H1. It seems most probable that the so-called ‘precursor’ events were not linked to the H events. However, if response to a common thermal forcing is considered, an increased response time of the LIS, up to ~2 kyr longer than the NWEIS, may be inferred. Negative salinity excursions of up to 2.6 indicate significant incursions of melt water associated with peaks in NWEIS instability. Decreased surface density led to a more stable stratification of the upper water column and is associated with reduced ventilation of intermediate waters, recorded in depleted epibenthic δ 13C (Cibicidoides wuellerstorfi). We suggest that instability and meltwater forcing of the NWEIS temporarily weakened Atlantic Meridional Overturning Circulation, allowing transient advance of southern-sourced waters to this site, prior to H events 1 and 2. [Copyright &y& Elsevier]