Onset of the icehouse world: Atlantic deep-water circulation during the Pliocene and Pleistocene
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Bell, David Benjamin
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The transition from the warm, “greenhouse” conditions of the Pliocene to the cold, “icehouse” conditions of the Pleistocene marks a significant development in climate history. The deep-ocean is the largest dynamic reservoir of heat and carbon dioxide in the climate system that is accessible on timescales of Plio-Pleistocene climate change. Therefore, changes in the state of the deep-ocean may have played an important role in large scale Plio-Pleistocene climate change via variability in the meridional overturning circulation of the Atlantic (AMOC). In this thesis, paleoceanographic reconstructions of Plio-Pleistocene Atlantic deep-water circulation are presented from the perspective of Ocean Drilling Program Sites 1264 (2505m depth) & 1267 (4350m depth), situated at ~30oS in the Southeast Atlantic. Reconstructions are based on high-resolution (~<5,000 year time-step), down-core measurements of oxygen (δ18O) and carbon (δ13C) stable isotope ratios in benthic foraminifera. During the Pliocene, widespread high δ13C values in the Atlantic and at Sites 1264 & 1267 indicate low nutrient conditions and active deep-water renewal. The early Pliocene closure of the Central American Seaway (CAS) (~4.7-4.2 Ma) is considered to have been influential in establishing strong deep-water formation in the North Atlantic. Evaluation of δ13O and δ13C records from Site 1264 and throughout the North Atlantic, however, indicate that the CAS closure event had only a limited impact outside of the Caribbean Basin. Meanwhile, during the interval ~3.6-2.7 Ma, δ13C-gradients between Sites 1264-1267 are near zero and suggest strong North Atlantic Deep Water (NADW) prevalence in the Southeast Atlantic, similar to or stronger than the modern situation. The transition into Pleistocene style glacial-interglacial cycles at ~2.7 Ma is associated with a reduction of NADW prevalence in the Atlantic, particularly during glacials and at depth. At ~2.4 Ma, δ18O and δ13C records from Sites 1264 & 1267 reveal marked changes in deep-water circulation. Large (>0.5‰) δ18O-gradients emerge, with heaviest values seen at Sites 1264 & 1267 compared to records from the North Atlantic. At the same time, δ13C values increase at Sites 1264 & 1267. The combination of high δ18O and δ13C values at Sites 1264 & 1267 is consistent with enhanced export of a dense component of NADW that enters the Atlantic from the Nordic Seas by spilling over the Iceland-Scotland Ridge. Comparisons with other North Atlantic records suggest that the pathway of Iceland-Scotland Overflow Water (ISOW) was restricted, flowing along the abyssal East Atlantic and piling up at Walvis Ridge. Between ~2.0-1.5 Ma, maximum δ13C values and minimum δ13Cgradients within the North Atlantic and between the North Atlantic and Sites 1264 & 1267 indicate that the overall export of NADW was strongest for the Pleistocene. After ~1.5 Ma, Atlantic δ18O-gradients begin to reduce, along with δ13C values, although δ13C-gradients still imply strong NADW export. Starting at ~1.3 Ma and across the Mid Pleistocene Transition (MPT), Atlantic δ18O-gradients reduced markedly, as did North Atlantic-Pacific δ18O-gradients but to a lesser degree. After ~0.9 Ma, glacial reductions in NADW presence at depth are the most severe of the entire Plio-Pleistocene, while interglacial export of NADW into the Atlantic remained almost as high as pre MPT conditions. Changes in the strength of AMOC during the Plio-Pleistocene are inferred through comparisons of Atlantic deep-water history with records of sea surface temperature from the high latitude North Atlantic, South Atlantic and North Pacific. I propose that AMOC played an important role in the evolution of Pleistocene climate. Enhanced northward heat transport, due to an increase in the strength of AMOC at ~2.4 Ma, limited the growth of continental ice sheets and sea ice within the North Atlantic region. This may have been caused by increased equator-to-pole thermal gradients and decreased atmospheric moisture transport, increasing salinity, as the global climate cooled. A strengthening of AMOC at ~2.4 Ma is paralleled by significant deep-water changes recorded at Sites 1264 & 1267, implicating enhanced ISOW export into the Southeast Atlantic as an important component of AMOC at this time. A maximum in AMOC occurred between ~2.0-1.5 Ma, along with warming in the mid latitude North Atlantic. Scavenged heat from the South Atlantic promoted enhanced cooling of the Southern Hemisphere and the expansion of sea ice at ~1.5 Ma. Feedbacks originating in the Southern Ocean then acted to cool the globe and eventually pre-condition the climate for the MPT.