The PIs' preliminary data from sediment cores from deep equatorial Pacific show evidence of old radiocarbon ages and thus the presence of old, poorly ventilated bottom water brought to the surface as deglaciation began at the end of the last glacial, thereby potentially explaining the prolonged decrease in radiocarbon content and a concurrent increase pCO2 levels of the atmosphere. The PIs propose to evaluate this ventilation signal further and test its significance through a combination of 14C and 230Th measurements on selected additional cores in order to better understand the effects of benthic processes. In addition, Thorium profiling will be used to test the significance of maxima in foraminiferal abundance that are dated with 14-C. This method will be used to evaluate the accumulation rates of benthic foraminifera and the possible role of winnowing and sediment focusing in creating benthic abundance peaks. Thorium data will also help refine chronology of millennial-scale events in the eastern Equatorial Pacific over the last 25 K.Y. Broader Impacts: The scientific impact of the findings could be strong for other paleoclimate research. Outreach and educational efforts include training of teachers, students and a post-doctoral researcher.
This was a collaborative project that sought to explore the radiocarbon age, biological productivity and sediment dynamics in the eastern equatorial Pacific (EEP) during the last glacial maximum (LGM) approximately 20,000 years ago. Our colleagues at the Woods Hole Oceanographic Institution (WHOI) led the radiocarbon portion of the study, and we at the Lamont-Doherty Earth Observatory of Columbia University (LDEO) led the studies of paleo-productivity and sedimentation. The sediments were analyzed for bulk sediment composition, and four of the study cores to determine the naturally occurring abundance in the sediments of a suite of isotopes of several elements, including uranium, thorium and protactinium. Uranium is relatively abundant in seawater, but is highly soluble, so it is only generally absent in sediment in the presence of oxygen. Elevated uranium concentrations occur only in the deepest section of one of the study cores, in sediment deposited prior to the LGM. Otherwise uranium was found in very low concentrations, indicating that the bottom waters and upper sediments at our EEP sites generally remained oxygenated throughout. Two isotopes of thorium were analyzed for this study. Thorium-230, produced by the decay of dissolved uranium in the water column and thorium-232, which comes entirely from land, typically as windblown dust in distant oceanic locations.. These isotopes were analyzed in each sample by ICP-MS, and the resulting data combine to provide a number of insights concerning the LGM conditions in the EEP. Calcium-carbonate burial and detrital fluxes were greater at the LGM in the EEP than today. This supports earlier indications that the preservation of calcium-carbonate was greater in the Pacific at this time due to chemical changes in the deep seawater. Dust fluxes generally declined since then, and in two locations underwent a sharp, rapid, decrease before reaching modern values. Although the shift was similar in both cores, it did not appear to occur simultaneously at the two locations, an observation that may also reveal changes in transport and deposition, although the results should first be confirmed. The opal flux was initially similar at all four sites and then subsequently diverged somewhat. Although not definitive, a net increase in LGM productivity in the EEP is not supported by these combined results. The accumulation of thorium-230 at all sites was typically slightly above its expected production rate. Each of the cores was characterized by a focusing factor (the ratio of observed-to-expected burial) of between 1.0 and 2.0, with only two brief intervals in a single core varying substantially above (~3.0) or below (~0.5) that range. This indicates that there has generally been net sediment accumulation at all of the sites, and that the local vertical rain of biological and inorganic particles constitutes the primary source of sediments at each. Large-scale lateral additions (focusing) or removal (winnowing) appear to be secondary or unimportant processes in the study cores. Previous evidence revealed the presence of multiple large peaks in the abundance of particular benthic foraminifera whose shells provide a primary means of dating past bottom waters by radiocarbon analysis. New results from this study reveal that those peaks in abundance are associated with peaks in the burial flux of the shells determined using the thorium-230 normalization method described above. Combined with the evidence for little-to-modest lateral influence at the study sites, this suggests that the abundance peaks are not artifacts of sediment dynamics, but instead were directly related to changes in benthic foraminifera’ productivity. Although biology on the seafloor is tied to the surface productivity for its supply of organic carbon and nutrients, the lack of an equivalent episodic signal of surface productivity implies that changes in the benthic ecosystem, rather than large-scale biological productivity at the surface, may have been responsible for the observed foramifera abundance variations. Protactinium-231, in its burial ratio to thorium-230 (Pa/Th), is an indicator of the abundance of settling particles to remove it from seawater to the bottom in locations like the deep Pacific Ocean where large-scale motions of seawater are muted. This study’s results reveal that Pa/Th consistently increased during the deglaciation and has been at highest levels recently in the EEP, with a minimum during the LGM. Because the Pa/Th burial ratio is driven by the particle rain at this location, and because that particle rain is predominantly biological, these data support the conclusion that despite the possibility of episodic variations in benthic foraminifera abundance and burial flux, the biological productivity in the eastern equatorial Pacific Ocean was lower during the LGM than more recently, and could not have contributed to the observed lower concentration of atmospheric CO2 at the LGM. Much of this work was accomplished by a postdoctoral investigator, who received laboratory and analytical training, scientific and professional mentoring, and introductions to new colleagues and opportunities. She is now leading the effort to publish these latest results.
