Showing papers by "Katharina Billups published in 2004"

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

Abstract: 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.

Cenozoic pelagic Sr/Ca records: Exploring a link to paleoproductivity

Abstract: [1] Recent studies have revealed that Sr/Ca ratios of coccolithophores may be affected by productivity. Here we compile published Sr/Ca data from bulk carbonate, fine fraction sediment, and planktonic foraminiferal Sr/Ca records that span the past 60 Myr and attempt to place these records into a paleoceanographic framework. We account for changes in seawater Sr/Ca ratios using the curve of Lear et al. [2003] and discuss our observations with respect to changes in the partitioning coefficient of Sr through time. We discuss limitations associated with postburial processes, temporal changes in the coccolith/planktonic foraminiferal rain ratio, and the coccolith species composition of the sediment. Despite these caveats, we are able to show that in the bulk and fine fraction carbonate records there are two broad periods of enhanced partitioning of Sr relative to today, the Oligocene, and the middle to late Miocene/early Pliocene. Because these are two intervals for which we can cite evidence for a relatively productive ocean on the regional or global scale, we believe that the Sr/Ca ratios of bulk carbonate can be explained, at least in part, by the effects of oceanic nutrient levels on Sr uptake during calcification of coccoliths, which make up the vast majority of these sediments. Within the limits of the inferred seawater Sr/Ca record the results of this study contribute a geologic perspective to recent laboratory and field studies that have raised the possibility that Sr incorporation into biogenic calcite is controlled by biogeochemical processes.

Millennial‐scale fluctuations in subtropical northwestern Atlantic surface ocean hydrography during the mid‐Pleistocene

Abstract: [1] We use planktonic foraminiferal (Globigerinoides sacculifer and Neogloboquadrina dutertrei) δ18O records from subtropical North Atlantic Site 1056 to reconstruct millennial-scale changes in sea surface hydrography during marine isotope stages (MIS) 10 through 12 (∼340–440 ka). The difference in the δ18O records provides a measure of the thermal gradient from the mixed layer to the seasonal thermocline (ΔTml-tc), which we use to constrain upper ocean stratification and infer northward ocean heat transport. Late Holocene G. sacculifer and N. dutertrei δ18O values place controls on how the foraminifera record recent ocean conditions at Site 1056. In the down core record we find extreme excursions toward G. sacculifer δ18O maxima and ΔTml-tc minima throughout the entire time interval. These cold events have a similar amplitude throughout but are more frequent during the glacial than the interglacial intervals. During interglacial MIS 11 the excursions provide evidence for repeated cooling of the sea surface contributing to existing evidence for relative climate instability during interglacial intervals. During MIS 10, millennial-scale variability in the subtropical record is very similar to that observed in the subpolar North Atlantic at Site 980, perhaps related to a common forcing such as a large Northern Hemisphere ice sheet. Within the limitation of the age models the cold events of MIS 11 are almost synchronous with benthic foraminiferal δ13C minima at subtropical North Atlantic Site 1063, which suggests a general surface to deep ocean link via thermohaline circulation during this interval of time.

Palaeoclimate: low-down on a rhythmic high.

Abstract: Changes in the amount of solar energy reaching Earth account for certain climate cycles at high and low latitudes. Surprisingly, the effects of a high-latitude cycle evidently reached into the tropics.