The Cariaco Basin, a deep, presently anoxic depression along the northern margin of Venezuela, has proven to be an important repository of paleoclimatic information by virtue of its rapidly accumulating, virtually undisturbed sediments. Focusing on the core with the longest sediment sequence currently available (ODP1002), this research will generate the first continuous decadal resolution marine record of climate and environmental change over the last ~550,000 years. The researchers employ XRF scanning to generate high-resolution geochemical records from Termination II (~140,000 years ago) back to the base of the recovered sequence, providing a cost-efficient and logical extension of high resolution XRF scanning already completed on the younger part of the core. The XRF data will provide insights into the rates and magnitudes of climate change on multiple timescales and over multiple glacial-interglacial cycles when natural climate forcing differed substantially. The data will shed light on tropical climate sensitivity on the timescale of human generations. Graduate and undergraduate students will be involved in the research.
Over the past two decades, the field of paleoclimatology has undergone a number of major paradigm shifts. One of these is a change in emphasis from studying the nature and forcing mechanisms associated with glacial-interglacial climate change to documenting multi-decadal to millennial-scale climate variability and determining its origin. Using support from this award, we generated an approximately 600,000 year record of paleoclimate change with decadal resolution from the western tropical Atlantic Ocean. The record comes from a 170-meter thick marine sediment sequence drilled by the Ocean Drilling Program (ODP) in the Cariaco Basin, a small basin located just off the northern Venezuelan coast. Cariaco Basin is unusual in that the deep water therein currently has no oxygen below a water depth of about 300 meters. These anoxic conditions preclude the presence of burrowing organisms that mix the sediment in most sea floor environments; hence sediments that accumulate in the deep basin today are characterized by fine layering that represents the seasonal contrast in settling particles. The climatology responsible for this fine seasonal layering is driven by migration of the Intertropical Convergence Zone (ITCZ) and associated variations in trade wind intensity and precipitation. During winter-spring, the ITCZ is in its most southerly position; rainfall is at a minimum and strong easterly winds cause intense upwelling and high productivity along the Venezuelan coast. As the ITCZ migrates north during the summer-fall, the trade winds diminish, upwelling ceases, and the rainy season begins. Among modern climatologists, the ITCZ is viewed as a yardstick of Atlantic climate variability since changes - primarily in precipitation - associated with shifts in the ITCZ have large and well-known societal and economic impacts in the region. The principal dataset produced in this project is a record of geochemical changes measured by scanning x-ray fluorescence (XRF) techniques at 0.5-cm intervals down the length of ODP Site 1002 in Cariaco Basin. Given the high sediment accumulation rates at Site 1002, this interval translates into measurements of elemental concentrations at approximately 15 year spacing. A key elemental record from the scanning-XRF dataset is that of molybdenum (Mo), an element highly sensitive to the presence of oxygen, which records a history of ventilation changes in the presently anoxic Cariaco Basin. Over the last glacial-interglacial cycle, Cariaco Mo closely mimics the record of air temperature preserved in Greenland ice cores, with high Mo and anoxic conditions occurring in Cariaco Basin during warm intervals (e.g., interstadials) over Greenland, and negligible Mo and oxygenated conditions prevalent when Greenland is cold (i.e., the Last Glacial Maximum and cold stadials). Greenland ice core temperature records and Cariaco Mo records can be mechanistically linked through changes in the position of the ITCZ during the last glacial period, with southward excursions during cold stadials limiting productivity and organic decomposition/oxygen consumption rates through decreases in riverine inputs as the rain belt associated with the ITCZ shifts southward. The longest ice core record from Greenland currently reaches back 123,000 years. In contrast, the Site 1002 record from Cariaco Basin spans multiple glacial-interglacial cycles. Assuming the close relationship between Cariaco Mo and Greenland temperature has persisted over time, patterns of earlier millennial-scale variability at high northern latitudes can be inferred. Our geochemical data on Mo and other elements confirm the increasing body of evidence that rapid climate changes are a common feature of Pleistocene climate, rather than an anomaly. In particular, the presence of fully oxygenated intervals during warm interglacials in the Cariaco record indicates that stadial-like southward excursions of the ITCZ can occur even during times of reduced ice sheet extent. This suggests that millennial-scale variability can be initiated by something other than ice-sheet related changes in the Northern Hemisphere. Our new record thus has implications for the role of high-latitude forcing of ITCZ position and tropical hydrology on millennial timescales. The results from this project will likely cause some reassessment of the causes and mechanisms that drive abrupt climate change, particularly as recorded in the tropics. The record generated from Cariaco Basin Site 1002 is the only marine record that currently exists in the tropics that has decadal resolution and that spans a time period as long as the last 600,000 years. As such it will be of interest as a record for comparison to other studies and for climate modelers interested in trying to simulate the observed variability. The data generated for this project formed a key part of the Ph.D. dissertation work of a female student who successfully defended in December 2012. Two undergraduate students gained research experience assisting with the collection of XRF data. Both students are now pursuing graduate degrees. Data generated by this study are being made publicly available upon publication at the World Data Center for Paleoclimatology at the National Geophysical Data Center in Boulder, Colorado.
