Katharina Billups

Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research

Thirteen laboratories from the USA and Europe participated in an intercomparison study of Mg/Ca and Sr/Ca measurements in foraminifera. The study included five planktonic species from surface sediments from different geographical regions and water depths. Each of the laboratories followed their own cleaning and analytical procedures and had no specific information about the samples. Analysis of solutions of known Mg/Ca and Sr/Ca ratios showed that the intralaboratory instrumental precision is better than 0.5% for both Mg/Ca and Sr/Ca measurements, regardless whether ICP-OES or ICP-MS is used. The interlaboratory precision on the analysis of standard solutions was about 1.5% and 0.9% for Mg/Ca and Sr/Ca measurements, respectively. These are equivalent to Mg/Ca-based temperature repeatability and reproducibility on the analysis of solutions of ±0.2°C and ±0.5°C, respectively. The analysis of foraminifera suggests an interlaboratory variance of about ±8% (%RSD) for Mg/Ca measurements, which translates to reproducibility of about ±2–3°C. The relatively large range in the reproducibility of foraminiferal analysis is primarily due to relatively poor intralaboratory repeatability (about ±1–2°C) and a bias (about 1°C) due to the application of different cleaning methods by different laboratories. Improving the consistency of cleaning methods among laboratories will, therefore, likely lead to better reproducibility. Even more importantly, the results of this study highlight the need for standards calibration among laboratories as a first step toward improving interlaboratory compatibility.

Application of benthic foraminiferal Mg/Ca ratios to questions of Cenozoic climate change

Abstract We investigate the evolution of Cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater (δ18Osw) derived from benthic foraminiferal Mg/Ca ratios to constrain the temperature effect contained in foraminiferal δ18O values. We have constructed two benthic foraminiferal Mg/Ca records from intermediate water depth sites (Ocean Drilling Program sites 757 and 689 from the subtropical Indian Ocean and the Weddell Sea, respectively). Together with the previously published composite record of Lear et al. [Science 287 (2002) 269–272] and the Neogene record from the Southern Ocean of Billups and Schrag [Paleoceanography 17 (2002) 10.1029/2000PA000567], we obtain three, almost complete representations of the δ18Osw for the past 52 Myr. We discuss the sensitivity of early Cenozoic Mg/Ca-derived paleotemperatures (and hence the δ18Osw) to assumptions about seawater Mg/Ca ratios. We find that during the middle Eocene (∼49–40 Ma), modern seawater ratios yield Mg/Ca-derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice-free conditions. Intermediate waters cooled during the middle Eocene reaching minimum temperatures by 40 Ma. The corresponding δ18Osw reconstructions support ice growth on Antarctica beginning by at least 40 Ma. At the Eocene/Oligocene boundary, Mg/Ca ratios (and hence temperatures) from Weddell Sea site 689 display a well-defined maximum. We caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of Mg in benthic foraminifera may be sensitive to factors other than temperature. Throughout the remainder of the Cenozoic, the temporal variability among δ18Osw records is similar and similar to longer-term trends in the benthic foraminiferal δ18O record. An exception occurs during the Pliocene when δ18Osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal δ18O records, which provides a new perspective to the ongoing debate about the stability of the Antarctic ice sheet. Maximum δ18Osw values recorded during the Pleistocene at Southern Ocean site 747 agree well with values derived from the geochemistry of pore waters [Schrag et al., Science 272 (1996) 1930–1932] further highlighting the value of the new Mg/Ca calibrations of Martin et al. [Earth Planet. Sci. Lett. 198 (2002) 193–209] and Lear et al. [Geochim. Cosmochim. Acta 66 (2002) 3375–3387] applied in this study. We conclude that the application of foraminiferal Mg/Ca ratios allows a refined view of Cenozoic ice volume history despite uncertainties related to the geochemical cycling of Mg and Ca on long time scales.

Late Miocene through early Pliocene deep water circulation and climate change viewed from the sub-Antarctic South Atlantic

Benthic foraminiferal stable isotope records for the past 11 Myr from a recently drilled site in the sub-Antarctic South Atlantic (site 1088, Ocean Drilling Program Leg 177, 41‡S, 15‡E, 2082 m water depth) provide, for the first time, a continuous long-term perspective on deep water distribution patterns and Southern Ocean climate change from the late Miocene through the early Pliocene. I have compiled published late Miocene through Pliocene stable isotope records to place the new South Atlantic record in a global framework. Carbon isotope gradients between the North Atlantic, South Atlantic, and Pacific indicate that a nutrient-depleted watermass, probably of North Atlantic origin, reached the sub-Antarctic South Atlantic after 6.6 Ma. By 6.0 Ma the relative proportion of the northernprovenance watermass was similar to today and by the early Pliocene it had increased to greater than the modern proportion suggesting that thermohaline overturn in the Atlantic was relatively strong prior to the early Pliocene interval of inferred climatic warmth. Site 1088 oxygen isotope values display a two-step increase between V7.4 Ma and 6.9 Ma, a trend that parallels a published N 18 O record of a site on the Atlantic coast of Morocco. This is perhaps best explained by a gradual cooling of watermasses that were sinking in the Southern Ocean. I speculate that relatively strong thermohaline overturn at rates comparable to the present day interglacial interval during the latest Miocene may have provided the initial conditions for early Pliocene climatic warmth. The impact of an emerging Central American Seaway on Atlantic^Pacific Ocean upper water exchange may have been felt in the North Atlantic beginning in the latest Miocene between 6.6 and 6.0 Ma, which would be V1.5 Myr earlier than previously thought. 9 2002 Elsevier Science B.V. All rights reserved.

Late Oligocene to early Miocene geochronology and paleoceanography from the subantarctic South Atlantic

Mi2 as well as several other distinctive isotope events. Our data suggest that the d 18 O maximum commonly associated with the Oligocene/Miocene (O/M) boundary falls within C6Cn.2r (23.86 Ma). The d 13 C maximum coincides, within the temporal resolution of our record, with C6Cn.2n/r boundary and hence to the O/M boundary. Comparison of the stable isotope record from Site 1090 to the orbitally tuned stable isotope record from ODP Site 929 across the O/M boundary shows that variability in the two records is very similar and can be correlated at and below the O/M boundary. Site 1090 stable isotope records also provide the first deep Southern Ocean end-member for reconstructions of circulation patterns and late Oligocene to early Miocene climate change. Comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow. Oxygen isotope gradients between sites suggest that the deep Southern Ocean was cold in comparison to the North Atlantic, Indian, and the Pacific Oceans. Dominance of cold Southern Component Deep Water at Site 1090, at least until 17 Ma, suggests that relatively cold circumpolar climatic conditions prevailed during the late Oligocene and early Miocene. We believe that a relatively cold Southern Ocean reflects unrestricted circumpolar flow through the Drake Passage in agreement with bathymetric reconstructions. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 1035 Geochemistry: Geochronology; 4870 Oceanography: Biological and Chemical: Stable isotopes; KEYWORDS: Paleoceanography; 1035 geochronology, stable isotopes

Link between oceanic heat transport, thermohaline circulation, and the Intertropical Convergence Zone in the early Pliocene Atlantic

Planktonic foraminiferal oxygen isotope records from the western and eastern tropical Pacific and Atlantic Oceans suggest a southward shift in the Intertropical Convergence Zone toward its modern location between 4.4 and 4.3 Ma. A concomitant shift in the carbon isotope compositions of Atlantic benthic foraminifera provides strong evidence for an increased thermohaline overturn at this time. We suggest that the southward shift of the Intertropical Convergence Zone and associated change in trade-wind circulation altered equatorial surface hydrography, increased the advection of warmer and more saline surface waters into the subtropical and North Atlantic, and contributed to thermohaline overturn.

Reconstructing the stable isotope geochemistry and paleotemperatures of the equatorial Atlantic during the last 150,000 years: Results from individual foraminifera

This study represents an attempt to extract paleoclimatic data from the deep-sea record by analyzing foraminiferal shells individually. Using the oxygen (δ18O) and carbon (δ13C) isotopic composition of individual Orbulina universa and Neogloboquadrina dutertrei, we present an approach to reconstruct the δ18O of seawater (δ18Ow), the δ13C of ΣCO2, and seasonal maximum sea surface temperatures (SST) in the western and eastern equatorial Atlantic. We examine the glacial and interglacial extremes of the last 150,000 years (isotope stages 1, 2, 5e, and 6). Comparison of recent water column hydrography with reconstructions from core top assemblages shows that O. universa and N. dutertrei δ18O and δ13C values accurately record hydrographic conditions in the mixed layer and upper thermocline at both sites. By analyzing shells individually, we can evaluate the effect of bioturbation on the range of δ18O and δ13C values in each interval and take it into consideration in our data interpretations. Downcore results show that N. dutertrei δ18O values in the western equatorial Atlantic reflect glacial to interglacial changes in δ18Ow due to continental ice formation (Δδ18O=1.30 ‰). We use changes in N. dutertrei δ18O values between core intervals to estimate the ice-volume effect in paleotemperature calculations for the mixed layer. To validate the use of O. universa for mixed layer reconstructions, we have added individual Globigerinoides sacculifer data for stages 1 and 2 at both sites. Paleotemperature reconstructions from O. universa δ18O values indicate that maximum seasonal mixed layer temperatures in the equatorial Atlantic decreased by at most 2.6°C between isotope stages 1 and 2 and by no more than 3.4°C between stages 1 and 6. Individual shell data from G. sacculifer yield similar results indicating that maximum O. universa δ18O values reflect the mixed layer environment. In agreement with Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) [1981] SST reconstructions for stage 2, these data indicate little change between glacial and interglacial paleotemperatures in the equatorial Atlantic. An electronic supplement of this material may be obtainedon a diskette or Anonymous FTP from KOSMOS.AGU.ORG.(LOGIN to AGUs FTP account using ANONYMOUS as theusername and GUEST as the password. Go to the rightdirectory by typing CD APEND. Type LS to see what files areavailable. Type GET and the name of the file to get it.Finally, type EXIT to leave the system.) (Paper 95PA03773,Reconstructing the stable isotope geochemistry andpaleotemperatures of the equatorial Atlantic during the last150,000 years: Results from individual foraminifera,Katharina Billups and Howard J. Spero). Diskette may beordered from American Geophysical union, 2000 FloridaAvenue, N. W., Washington, DC 20009;

Early Pliocene climate: A perspective from the western equatorial Atlantic Warm Pool

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Surface ocean density gradients during the Last Glacial Maximum

High-resolution (∼3–4 kyr) planktonic stable isotope stratigraphies from Site 925 drilled on Ceara Rise (Ocean Drilling Program Leg 154) are used to investigate the role of the western equatorial Atlantic sea surface hydrography in early Pliocene (3.2–4.7 Ma) climate change. Oxygen isotope results from Globigerinoides sacculifer, a mixed layer dweller, suggest that equatorial sea surface temperatures were cooler than today by ∼2°–3°C, consistent with relatively strong northward advection of heat away from the equator. Oxygen isotope results from Neogloboquadrina dutertrei suggest that over the long term this thermocline dweller tracks global ice volume fluctuations. Stable isotope gradients between the two planktonic species throughout the entire interval imply a stable warm pool in the western equatorial Atlantic. We observe a rapid (3.4 kyr) decrease in G. sacculifer and the N. dutertrei δ18O values at 4.36 Ma that may reflect a freshening of the sea surface, a direct response to a southward displacement of the Intertropical Convergence Zone, and perhaps, an indirect response to restricted flow through the Central American Seaway.

The intensification of northern component deepwater formation during the mid‐Pleistocene climate transition

We have compiled published planktonic foraminiferal oxygen isotope values from the tropical to subtropical Atlantic and Pacific for the late Holocene and Last Glacial Maximum (LGM). We interpret the spatial distribution of δ18O values in terms of meridional density gradients in the LGM surface ocean and discuss implications on tropical ocean thermocline structure. In both oceans, glacial δ18O values display a pattern of interhemispheric asymmetry evidenced by increasingly higher glacial δ18O values toward northern latitudes. The δ18O-derived density estimates suggest that the northern subtropical surface ocean density was higher during the LGM than today, which has significant implications on LGM thermocline ventilation. In the Atlantic, steeper density gradients in the northern subtropics during the LGM are consistent with a more compressed subtropical gyre and thermocline water formation farther south than today. In the Pacific the steeper density gradients suggest a significant reduction in freshwater fluxes to the northern tropical and subtropical regions, which raises the possibility of a more significant component of Northern Hemisphere source waters to the tropical Pacific thermocline. These predictions could be tested with numerical ocean models that assess the structure of the tropical thermocline in the glacial ocean.

Origin of millennial‐scale climate signals in the subtropical North Atlantic

We reconstruct mid-Pleistocene (marine isotope stages (MISs) 13–18) deepwater hydrography at Ocean Drilling Program Site 1063 (4583 m water depth, subtropical North Atlantic) using benthic foraminiferal stable isotope records. These new records complete an ~900 kyr long stratigraphy spanning MISs 8–29 (~250–1030 Ka) when combined with previously published records from Site 1063. The results indicate a change in the circulation regime of the abyssal subtropical North Atlantic during MIS 17. Prior to MIS 17, no significant glacial or interglacial δ13C gradients are evident between Site 1063 and the deep South Atlantic. After MIS 17, interglacial intervals at Site 1063 are characterized by δ13C values that consistently approach those recorded in the deep North Atlantic. Comparing Site 1063 δ13C values to 26 additional published records throughout the entire Atlantic basin supports the idea that this δ13C increase is unique to the deep North Atlantic. After MIS 17, the basin-wide influence of higher δ13C values suggests an increased relative flux of northern sourced bottom waters during interglacial periods. The timing of northern sourced water influence at Site 1063 is consistent with the timing of a shift in the orientation of the Arctic Front. Thus, this shift may signify a link between the northward penetration of relatively warm, saline surface waters into the Norwegian-Greenland Seas stimulating deep convection. Our findings fit well with the model of Imbrie et al. (1993) for the importance of the Nordic heat pump in establishing strong 100 kyr cyclicity in late Pleistocene glacial cycles.