Changing export production in the Eastern Equatorial Pacific, 160 ka to present Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m326m448z

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  • The increase in modern atmospheric CO₂ concentration (360 ppm) from pre-industrial levels (280 ppm) raises concern over the impact of anthropogenic carbon dioxide input on atmospheric chemistry. For this reason, it is important to understand the natural fluctuations of atmospheric carbon dioxide in the past. The Vostok ice core record of atmospheric CO₂ is highly variable through time with the lowest values occurring during glacial intervals. Changes in export production to the deep ocean could provide at least part of the answer to explain the low glacial values. Here we present evidence for changes in primary productivity over the past 160 ka based on four export production proxies, organic carbon mass accumulation rate (MAR), benthic foraminiferal assemblages, a carbon isotope gradient responding to local changes in the organic carbon flux to the sea floor, and a carbon isotope gradient responding to regional changes in organic carbon flux over the Eastern Equatorial Pacific (EEP). The four surface production proxies were applied to three sites in the EEP, ME0005A-27JC (1.55° 5, 82.77° W), ME0005A- 24JC (0.02° N, 86.46° W), and Y69-71(0.8° N, 86.48° W). Organic carbon MAR results were similar to other studies using biogenic MAR proxies in the EEP. The proxy peaked during transitions, mostly from glacial to interglacial. We also observe a large MAR spike during MIS 2, similar to findings of Pedersen (1991). The benthic foraminiferal abundance data set obtained from Loubere (1999a) was hindered by a no analog problem and compels a reanalysis of the results from past studies. The local carbon isotope proxy appears to be more successful at recording changes in short-term organic carbon flux variability rather than changes on longer time scales, e.g. glacial/interglacial cycles. The proxy is also hampered by a decoupling of the two benthic foraminiferal species under an environment of both extreme organic carbon flux and low bottom water oxygen. The regional proxy results suggest the EEP did experience regional shifts in organic carbon flux through the last l6Oky. The proxy records larger fluxes of organic carbon inside the Panama Basin relative to the location outside the basin during glacial episodes. The position of the two cores in this study was not ideal for this proxy through the entire record. The proxy was limited by an imperfect monitor of source waters. From this analysis, it is obvious that primary production can not be easily resolved in the EEP over the last 160 ky. The results obtained from these analyses suggest a closer examination of how export production proxies are being interpreted and have been interpreted in past work. Thus, it was not possible to resolve the question of surface production as a factor controlling low glacial atmospheric CO₂ values. It is imperative to remedy the current status of the proxies presented in this project. This could be accomplished by examining surface production proxies in the modern environment to improve our understanding of how each proxy is related to organic carbon flux. It is obvious from this study and past work that the EEP is a complex region of the ocean. It appears the region is dominated by changes in deep and surface water circulation fluctuations through time. Future work is necessary to gain a more comprehensive view of regional dynamics of the EEP in order to resolve the issue of changes in primary production and ultimately atmospheric CO₂ over the past 160 ky.
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