Recent scientific investigations of sub-millennial paleoceanographic variability have attempted to use the population statistics of single planktic foraminiferal δ18O in an attempt to characterize the variability of high-frequency signals such as the El Niño Southern Oscillation (ENSO). However, this approach is complicated by the relatively short lifespan of individual foraminifera (~2-4 weeks) compared to the time represented by a sediment sample of a marine core (decades to millennia). The resolving ability of individual foraminiferal analyses (IFA) is investigated through simulations on an idealized virtual sediment sample. We focus on ENSO-related sea-surface temperatures (SST) anomalies in the tropical Pacific Ocean (Niño3.4 region). We constrain uncertainties on the range and standard deviation associated with the IFA technique using a bootstrap Monte Carlo approach. Sensitivity to changes in ENSO amplitude and frequency and the influence of the seasonal cycle on IFA are investigated through the construction of synthetic time series containing different characteristics of variability. We find that the standard deviation and range of the population of individual foraminiferal δ18O may be used to detect ENSO amplitude changes at particular thresholds (though the uncertainty in range is much larger than in standard deviation); however, it is highly improbable that IFA can resolve changes in ENSO frequency. We also determine that the main driver of the IFA signal is ENSO amplitude as opposed to changes in the seasonal cycle although this is specific to Niño3.4 where the SST response to ENSO is maximal. Our results suggest that rigorous uncertainty analysis is crucial to the proper interpretation of IFA data and should become a standard in individual foraminiferal studies. / text

Geological records of past sea ice, such as those contained in Arctic marine sediments, offer an opportunity to strengthen our understanding of long-term sea ice variability, provided unambiguous paleo-sea ice proxies can be developed. One such recently proposed proxy is IP₂₅, a highly branched isoprenoid alkene biosynthesized exclusively by sea-ice dwelling diatoms (Haslea spp.), which is well preserved in marine sediments and could be used to reconstruct past changes in spring sea-ice extent. However, little is known about regional-scale controls on IP₂₅ production in sea ice, limiting its wider applicability as a paleo-sea-ice proxy. To address this issue we examined the distributions of IP₂₅ and the marine productivity biomarkers dinosterol and brassicasterol in a suite of surface sediment samples distributed across the Arctic. We find a statistically significant, logarithmic relationship between IP₂₅ and spring sea ice cover in samples from arctic and subarctic sites in the Pacific (n = 96, r² = 0.67, P < 0.0001) and the Atlantic n = 25, r² = 0.50, P < 0.0001), though the absolute concentrations of IP₂₅ are generally higher in the Atlantic (1.6 - 166.4 [mu]g/g OC) than in the Pacific (0 - 38.5 [mu]g/g OC) for equivalent sea-ice cover, and there are regional and basin-specific differences in the slope of the IP₂₅ - sea ice relationship. After normalization of IP₂₅ concentrations to that of a productivity biomarker (e.g., dinosterol; the PDIP₂₅ index) the proxy-sea ice relationship in greatly improved for all regions (r² = 0.86 and r² = 0.75 for Atlantic and Pacific, respectively) and most of the basin specific differences in the rate of change of IP₂₅ with sea ice are removed. This suggests that productivity plays an important secondary role in controlling IP₂₅ concentrations. However, the use of the PDIP₂₅ index does not change the absolute differences in concentrations seen in the Atlantic and the Pacific, and previously published data from Fram Strait remain anomalous when compared to the rest of our data. This suggests that there are additional, yet unidentified controls on the IP₂₅ proxy - sea ice relationship, which may hinder the development of an Arctic-wide calibration but that the PDIP₂₅ index is a viable tool for local and regional sea ice reconstructions. / text

Whether a global greenhouse interval is a distinct or distant future, it is important to understand the dynamics of a greenhouse system. During such intervals the oceans, in the absence of sizeable polar ice caps, flood the continental shelf. The stratification and circulation of these epicontinental seas are open to debate, because there are no Recent analogs. The carbon and oxygen stable isotope record of fossil molluscan shell from epicontinental seas has the potential to reveal their stratification and seasonal cycles. Whether a global greenhouse interval is a distinct or distant future, it is important to understand the dynamics of a greenhouse system. During such intervals the oceans, in the absence of sizeable polar ice caps, flood the continental shelf. The stratification and circulation of these epicontinental seas are open to debate, because there are no Recent analogs. The carbon and oxygen stable isotope record of fossil molluscan shell from epicontinental seas has the potential to reveal their stratification and seasonal cycles. As a study sample, mollusks from the Baculites compressus and Baculites cuneatus biozones of the Western Interior Seaway of North America were collected from three locations: Kremmling, Colorado; Trask Ranch, South Dakota; Game Ranch, South Dakota. These fossils date to the Campanian (Late Cretaceous). Taxa include ammonites, bivalves, gastropods, and nautiloids. The first part of this investigation, described in Chapter 2, investigates the degree of alteration in these specimens. Elevated concentrations of minor elements such as magnesium and strontium reveal alteration from the original aragonite and/or calcite skeletons. Concentrations of these elements obtained by ICP-OES analysis are compared within several suites of specimens: mode of preservation, shell testing location, shell color, cementation, appearance under light microscope, and appearance under scanning electron microscope. Each of these suites tests a hypothesis about optimal shell preservation. Shell was found to be preserved best in shale rather than concretions, ammonite phragmacone rather than septa, opalescent specimens rather that nonopalescent ones, and uncemented shells rather than cemented shells, especially those with second-order versus first-order cement. Salinity and temperature values were derived for the organisms in the Western Interior Seaway: while bivalves produced unusually low temperatures, the others were reasonable for an inland sea. The second part of this study, described in Chapter 3, examines the isotopic record within exemplary mollusk shells, taken perpendicular to growth lines. The data for this investigation in sclerochronology documents the dominant isotopically enigmatic bottom-water habitat of the Inoceramus, the geochemical signature of the overlying water mass inhabited by Baculites, and short-term migrations between the two water masses in the nautiloid Eutrephoceras.

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