This research provides the first Pliocene paleo-oceanographic observations from the Bering Sea and supports the objectives of IODP expedition 323 for understanding the associated factors influencing climate change during the Pliocene and Pleistocene, including a warm period of the early Pliocene, when pCO2 levels were similar to today¡¦s levels. The flow of water masses between the Bering Sea and the Arctic and Pacific oceans, changes in Arctic ice volume, and whether dense intermediate waters formed in the Pacific during this time will be investigated as some of the possible triggers of past climate change. The high resolution sediment records obtained from this cruise, with relatively large-amplitude signals, provide a unique opportunity to understand how insulation changes in the upper atmosphere propagate through the climate system and to further test if the orbital cycles of climate change primarily operate on 23ky versus 40 ky timescales. This study utilizes K % logging data from the cruise with paleomagnetic, sedimentological (eg., grain size analysis) and paleontological (eg., analysis of microfossil assemblages; d18O and d13C measurements of benthic and planktonic forams) measurements from the drill cores. The information expected from this study is critical for understanding the fundamental drivers and feedbacks between ice sheets, oceanic and atmospheric circulation and the global carbon cycle.
Broader Impacts: This project significantly advances large investments previously made in an important IODP expedition. The data derived from this investigation can provide important information for paleoclimate/ocean models, and is relevant towards projections of future climate under IPCC AR4 scenarios. The project includes a team of early and mid-career scientists, and includes international collaborations with other scientists of the IODP community. Support is also provided for undergraduate and graduate education, and for public outreach and education.
Our paleomagnetic and rock magnetic studies of deep-sea sediments from IODP Ex. 323 in the Bering Sea have focused on two problems. The first problem is to date and correlate the sediments from 7 sites in the Bering Sea with as high a resolution as possible over the last 1,000,000 years. The second problem has been to use rock magnetic variability as a tool in understanding the physical sedimentation processes active over the Bering Sea in this time interval and determining what climatic/environmental processes caused variations in the sedimentation processes. We have u-channel sampled 300 m of IODP Expedition 323 sediments that are younger than 1,000,000 years in age for paleomagnetic and rock magnetic studies. Our goal was to develop a paleomagnetic chronostratigraphy that can be used, with paleobiological and isotope age estimates, to build a detailed space/time framework of Bering Sea deep-sea sedimentation. We have successfully tested the replication between u-channel and shipboard measurements. We have also carried out more detailed u-channel paleomagnetic and rock magnetic measurements on selected cores. Our detailed u-channel magnetic measurements corroborate our shipboard placement of the Brunhes/Matuyama boundary at Sites 1341 and 1343. We have identified one magnetic field excursion that is recorded in sediments of Sites 1339, 1343, 1344, and 1345(?). We think that this is Excursion 7a (190 ka), which is also seen in the high-latitude North Atlantic Ocean (Channell et al., 1999). We have also identified several intervals of distinctive paleomagnetic secular variation (PSV) that appear to be correlatable among Sites 1343, 1344, and 1345. These results have permitted us to provide a paleomagnetic chronostratigraphy and regional correlation scheme among the IODP Expedition 323 sites that will be used by other scientists interested in the paleoclimate/paleoenvironment history of these sediments. IODP Expedition 323 shipboard rock magnetic measurements identified a dramatic bimodal character to the sediments, alternating between sediments with strong natural magnetic remanence (NRM) and magnetic susceptibility (chi) and those with order-of-magnitude lower values. The low intensity intervals were initially interpreted to be associated with biogenic oozes that caused significant magnetic mineral dissolution. We have made long-core NRM and ARM u-channel measurements on selected cores and cut up selected cores to carry out more detailed rock magnetic measurements at a 2 cm resolution. It is clear that the NRM undergoes significant grain size changes that are predominantly due to environmental changes in sediment supply and flux. We do see modest evidence for some degree of magnetic mineral dissolution (biased to finer grains) below the uppermost 1 m at several sites. But, it turns out most of the sediments are too coarse (silts) for much magnetic mineral dissolution. We have made magnetic separates at several depths in the uppermost 10 m of Site U1345 to assess magnetic mineralogy and evidence for early sediment diagenesis. The primary magnetic mineral at all levels is detrital magnetite. Our key finding, however, is that the bimodal pattern of magnetic susceptibility is due to fundamental changes in the processes of deep-sea sedimentation during glacial versus interglacial conditions. Glacials have finer-grained sediments (mean grain size of ~20 microns, fine silt) while interglacials have coarser-grained sediments (mean grain size of 50 microns, coarse silt). The changing grain size is related to flux of coarse continental sediment off the shelf in interglacial times, and finer clastic flux moved by intermediate/deep-ocean circulation during glacials (when low sea level and continental glaciers and permanent ice limit the movement of coarse continental sediments). Our results show evidence for significant millennial-scale environmental variability in Bering-Sea deep-sea sedimentation during both glacials and interglacials.
