Jimin Yu

B/Ca in planktonic foraminifera as a proxy for surface seawater pH

Boron isotope systematics indicate that boron incorporation into foraminiferal CaCO3 is determined by the partition coefficient, K D (=

Loss of carbon from the deep sea since the last glacial maximum

\frac{\left[{\rm B}/{\rm Ca}\right]{\rm CaCO3}}{\left[{\rm B} \left({\rm OH}\right)4- /{\rm HCO}3-\right]\rm seawater), and [B(OH)4 -/HCO3 -]seawater, providing, in principle, a method to estimate seawater pH and PCO2. We have measured B/Ca ratios in Globigerina bulloides and Globorotalia inflata for a series of core tops from the North Atlantic and the Southern Ocean and in Globigerinoides ruber (white) from Ocean Drilling Program (ODP) site 668B on the Sierra Leone Rise in the eastern equatorial Atlantic. B/Ca ratios in these species of planktonic foraminifera seem unaffected by dissolution on the seafloor. K D shows a strong species-specific dependence on calcification temperature, which can be corrected for using the Mg/Ca temperature proxy. A preliminary study of G. inflata from Southern Ocean sediment core CHAT 16K suggests that temperature-corrected B/Ca was ~30% higher during the last glacial. Correspondingly, pH was 0.15 units higher and aqueous PCO2 was 95 μatm lower at this site at the Last Glacial Maximum. The covariation between reconstructed PCO2 and the atmospheric pCO2 from the Vostok ice core demonstrates the feasibility of using B/Ca in planktonic foraminifera for reconstructing past variations in pH and PCO2.

Determination of multiple element/calcium ratios in foraminiferal calcite by quadrupole ICP-MS

Moving Carbon During the last glacial maximum, approximately 23,000 years ago, both the atmosphere and the terrestrial biosphere contained much less carbon than in the immediately preindustrial era. The carbon must have been stored in the deep ocean, and the transfer of carbon to the air and land during deglaciation must have affected the carbonate chemistry and carbon isotopic composition of the sea. Yu et al. (p. 1084) estimated how deep-water carbonate concentrations changed over the course of the last deglaciation and combined their results with 13C/12C data to show that carbon released by the deep ocean between 17.5 and 14.5 thousand years ago mostly stayed in the atmosphere as CO2, while between 14 and 10 thousand years ago, a substantial fraction was absorbed by the terrestrial biosphere. Carbon loss from the ocean to the atmosphere and terrestrial biosphere occurred at different rates in the last deglaciation. Deep-ocean carbonate ion concentrations ([CO32–]) and carbon isotopic ratios (δ13C) place important constraints on past redistributions of carbon in the ocean-land-atmosphere system and hence provide clues to the causes of atmospheric CO2 concentration changes. However, existing deep-sea [CO32–] reconstructions conflict with one another, complicating paleoceanographic interpretations. Here, we present deep-sea [CO32–] for five cores from the three major oceans quantified using benthic foraminiferal boron/calcium ratios since the last glacial period. Combined benthic δ13C and [CO32–] results indicate that deep-sea-released CO2 during the early deglacial period (17.5 to 14.5 thousand years ago) was preferentially stored in the atmosphere, whereas during the late deglacial period (14 to 10 thousand years ago), besides contributing to the contemporary atmospheric CO2 rise, a substantial portion of CO2 released from oceans was absorbed by the terrestrial biosphere.

Preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning

A method has been developed for rapid and precise simultaneous determination of nine element/Ca ratios in foraminiferal tests directly from intensity ratios using external, matrix-matched standards on a quadrupole inductively coupled plasma–mass spectrometer (ICP-MS). All quantification isotopes are determined in pulse mode to avoid cross-calibration. Small argide (40Ar26Mg) interferences on 66Zn are corrected by using two additional Mg and Zn standards. A stable signal, conducive for high-precision measurements, is obtained by cone conditioning. Variable calcium concentration has negligible effect on Li, Al, Mn, and Sr, but Ca concentrations for standards and samples need to be constrained at a similar level for precise measurements of Zn, Cd, and U. Aliquots of samples are first analyzed for Ca concentrations on an inductively coupled plasma–atomic emission spectrometer (ICP-AES), and the remaining solutions are diluted to Ca concentration of 100 ppm for ratio measurements to assure data quality. The long-term reproducibility of the method yielded precisions of Li/Ca = 2.4%, B/Ca = 4.2%, Mg/Ca = 1.4%, Al/Ca = 14%, Mn/Ca = 0.9%, Zn/Ca = 2.8% (1.2∼7.8 μmol/mol) and 5.1% (0.5∼1.2 μmol/mol), Sr/Ca = 0.9%, Cd/Ca = 2.4% (0.07∼0.24 μmol/mol) and 4.8% (0.01∼0.07 μmol/mol), and U/Ca = 2.5% for foraminiferal samples as small as 60 μg.

Bipolar seesaw control on last interglacial sea level

We have investigated the effect of cleaning procedures on eight element/Ca ratios in three widely used benthic foraminifera species from core top and down core sediments. Two cleaning techniques were employed: (1) comparison between “Mg-cleaning” and “Cd-cleaning” methods and (2) comparison between the various constituent reagents. Li/Ca, B/Ca, and Sr/Ca ratios remained unchanged for samples subjected to different treatments, but Mg/Ca, Mn/Ca, Zn/Ca, Cd/Ca, and U/Ca were substantially decreased when foraminifera shells were cleaned with reagents containing citrate. In contrast, no significant decreases in element/Ca ratios were observed for samples cleaned in N2H4 without citrate, indicating that the role of N2H4 was insignificant during reductive cleaning and that citrate is responsible for decreases of Mg/Ca, Mn/Ca, Zn/Ca, Cd/Ca, and U/Ca. Decreases in these ratios are most likely due to (1) removal of contaminant particles enriched in Mg, Mn, Zn, Cd and U and/or (2) preferential leaching of CaCO3 by the formation of stable metal complexes through chelation between citrate and metals heterogeneously distributed in foraminiferal shells. Due to the negligible effect of N2H4 and preferential dissolution of CaCO3 during reductive cleaning, it is suggested that the reductive cleaning step should be omitted and the “Mg-cleaning” method [Barker et al., 2003] could be employed to clean foraminiferal shells for trace element measurements.

A warm and poorly ventilated deep Arctic Mediterranean during the last glacial period

Our current understanding of ocean–atmosphere–cryosphere interactions at ice-age terminations relies largely on assessments of the most recent (last) glacial–interglacial transition, Termination I (T-I). But the extent to which T-I is representative of previous terminations remains unclear. Testing the consistency of termination processes requires comparison of time series of critical climate parameters with detailed absolute and relative age control. However, such age control has been lacking for even the penultimate glacial termination (T-II), which culminated in a sea-level highstand during the last interglacial period that was several metres above present. Here we show that Heinrich Stadial 11 (HS11), a prominent North Atlantic cold episode, occurred between 135 ± 1 and 130 ± 2 thousand years ago and was linked with rapid sea-level rise during T-II. Our conclusions are based on new and existing data for T-II and the last interglacial that we collate onto a single, radiometrically constrained chronology. The HS11 cold episode punctuated T-II and coincided directly with a major deglacial meltwater pulse, which predominantly entered the North Atlantic Ocean and accounted for about 70 per cent of the glacial–interglacial sea-level rise. We conclude that, possibly in response to stronger insolation and CO2 forcing earlier in T-II, the relationship between climate and ice-volume changes differed fundamentally from that of T-I. In T-I, the major sea-level rise clearly post-dates Heinrich Stadial 1. We also find that HS11 coincided with sustained Antarctic warming, probably through a bipolar seesaw temperature response, and propose that this heat gain at high southern latitudes promoted Antarctic ice-sheet melting that fuelled the last interglacial sea-level peak.

Seasonal contributions of catchment weathering and eolian dust to river water chemistry, northeastern Tibetan Plateau: Chemical and Sr isotopic constraints

Slow circulation in the cold Arctic The Arctic Ocean and Nordic Seas together supply dense, sinking water to the Atlantic Meridional Overturning Circulation (AMOC). The redistribution of heat by the AMOC, in turn, exerts a major influence on climate in the Northern Hemisphere. Thornalley et al. report that during the last glacial period, those regions were nearly stagnant and supplied almost none of the water that they presently contribute to the AMOC. This low rate of flow into the Atlantic was probably due to an absence of vigorous deep-water formation in the Arctic Mediterranean as a consequence of the extensive ice cover there at that time. Science, this issue p. 706 Deep-water formation in some Arctic seas nearly ceased during the peak of the last glacial period. Changes in the formation of dense water in the Arctic Ocean and Nordic Seas [the “Arctic Mediterranean” (AM)] probably contributed to the altered climate of the last glacial period. We examined past changes in AM circulation by reconstructing radiocarbon ventilation ages of the deep Nordic Seas over the past 30,000 years. Our results show that the glacial deep AM was extremely poorly ventilated (ventilation ages of up to 10,000 years). Subsequent episodic overflow of aged water into the mid-depth North Atlantic occurred during deglaciation. Proxy data also suggest that the deep glacial AM was ~2° to 3°C warmer than modern temperatures; deglacial mixing of the deep AM with the upper ocean thus potentially contributed to the melting of sea ice, icebergs, and terminal ice-sheet margins.

Comment on “Deep-Sea Temperature and Ice Volume Changes Across the Pliocene-Pleistocene Climate Transitions”

relative to winter. It is noticeable that both the lowest and the highest 87 Sr/ 86 Sr values of the Buha River waters occurred in the monsoon season, indicating a sensitive response of carbonate versus silicate weathering sources to hydrological forcing on a seasonal basis. A significant decrease in Na/cation, together with lower Sr isotope ratios, is consistent with a greater proportion of carbonate weathering relative to silicate weathering in the early monsoon season. High temperature and increased rainfall during the peak of the monsoon facilitate an increased proportion of ions derived from silicates, partly from groundwaters, to river water. In other seasons, elemental and 87 Sr/ 86 Sr ratios vary much less, indicating a constant ratio of silicate to carbonate weathering, consistent with limited variation in discharge. Our results highlight that in a semiarid region where climatic conditions vary seasonally, in addition to silicate and carbonate contributions, supply from eolian dust may also play a significant role in controlling seasonal variations in chemistry of river waters.

Shifting ocean carbonate chemistry during the Eocene-Oligocene climate transition: Implications for deep-ocean Mg/Ca paleothermometry

Sosdian and Rosenthal (Reports, 17 July 2009, p. 306) used magnesium/calcium ratios in benthic foraminifera from the North Atlantic to reconstruct past bottom-water temperatures. They suggested that both ice volume change and ice-sheet dynamics played important roles during the late Pliocene and mid-Pleistocene climate transitions. We present evidence that their record of deep ocean temperature is not reliable, thus raising doubts about their conclusions.

Poorly ventilated deep ocean at the Last Glacial Maximum inferred from carbon isotopes: A data‐model comparison study

To date, no conclusive evidence has been identified for intermediate or deep water cooling associated with the > 1 parts per thousand benthic delta O-18 increase at the Eocene-Oligocene transition (EOT) when large permanent ice sheets first appeared on Antarctica. Interpretation of this isotopic shift as purely ice volume change necessitates bipolar glaciation in the early Oligocene approaching that of the Last Glacial Maximum. To test this hypothesis, it is necessary to have knowledge about deep water temperature, which previous studies have attempted to reconstruct using benthic foraminiferal Mg/Ca ratios. However, it appears likely that contemporaneous changes in ocean carbonate chemistry compromised the Mg/Ca temperature sensitivity of benthic foraminifera at deep sites. New geochemical proxy records from a relatively shallow core, ODP Site 1263 (estimated paleodepth of 2100 m on the Walvis Ridge), reveal that carbonate chemistry change across the EOT was not limited to deep sites but extended well above the lysocline, critically limiting our ability to obtain reliable estimates of deep-ocean cooling during that time. Benthic Li/Ca measurements, used as a proxy for [CO32-], suggest that [CO32-] increased by similar to 29 mu mol/kg at Site 1263 across the EOT and likely impacted benthic foraminiferal Mg/Ca. A [CO32-]-benthic Mg/Ca relationship is most apparent during the early EOT when the overall increase in [CO32-] is interrupted by an apparent dissolution event. Planktonic d18O and Mg/Ca records suggest no change in thermocline temperature and a delta O-18(seawater) increase of up to 0.6 parts per thousand at this site across the EOT, consistent with previous estimates and supporting the absence of extensive bipolar glaciation in the early Oligocene.