Investigations of paleoclimate variations using accelerator mass spectrometry [report]

J R Southon, M Kashgarian, T A Brown
2000 unpublished
Introduction This project has used Accelerator Mass Spectrometry (AMS) 14C measurements to study climate and carbon cycle variations on time scales from decades to millennia over the past 30,000 years, primarily in the western US and the North Pacific. 14C dates provide a temporal framework for records of climate change, and natural radiocarbon acts as a carbon cycle tracer in independently dated records. The overall basis for the study is the observation that attempts to model future climate
more » ... el future climate and carbon cycle changes cannot be taken seriously if the models have not been adequately tested. Paleoclimate studies are unique because they provide realistic test data under climate conditions significantly different from those of the present, whereas instrumental results can only sample the system as it is today. The aim of this project has been to better establish the extent, timing, and causes of past climate perturbations, and the carbon cycle changes with which they are linked. This provides real-world data for model testing, both for the development of individual models and also for inter-model diagnosis and comparison activities such as those of LLNL's P O I program; it helps us achieve a better basic understanding of how the climate system works so that models can be improved; and it gives an indication of the natural variability in the climate system underlying any anthropogenically-driven changes. The research has involved four projects which test hypotheses concerning the overall behaviour of the North Pacific climate system. All are aspects of an overall theme that climate linkages are strong and direct, so that regional climate records are correlated, details of fine structure are important, and accurate and precise dating is critical for establishing correlations and even causality. An important requirement for such studies is the requirement for an accurate and precise radiocarbon calibration, to allow better correlation of radiocarbon-dated records with calendric paleoclimate archives such as ice cores. The extension of the radiocarbon calibration back into the late Pleistocene (the period of deglaciation) thus constitutes a fifth project. This project has been Institute-oriented in that it was only possible through collaborations with researchers from several UC campuses, and other US and foreign institutions. These collaborators have provided expertise in sampling and access to the best available paleoclimate records. In turn, CAMS scientists have provided expertise in selecting the best samples for 14C measurements, and in interpreting radiocarbon results in terms of climate and carbon cycle changes, plus access to unmatched 1% measurement capabilities. Project output has included climate model test data plus fundamental information on the carbon cycle and the climate system, assisting LLNL modelers (and the modeling community as a whole) to improve their simulations. The scope of this project, and the widespread collaborations involved, have enhanced the visibility of LLNL as a focus for global climate research. This has in part led to discussions with NSF program managers on how to make LLNL researchers eligible to take part as PI'S in NSF's Earth System History (ESH) program. For at least some proposal callsjoint ESH/NOAA programsit appears that submission via N O M will be possible. An initial proposal by one of us (MK) to fund coral research based on the results from an LDRD which was spun off from this one as a separate project in 1998 (see below), has been submitted under this arrangement. Scientific and technical programs 1. Benthic-planktonic age differences: Pleistocene vs Holocene. There are significant differences in the radiocarbon ages for carbonate from coeval surface-and bottom-dwelling organisms (foraminifera) preserved in ocean sediments. These offsets show the apparent age of the local bottom water relative to the surface, at the time the organisms lived. Research at CAMS and elsewhere, using these benthic-planktonic age differences as an ocean circulation tracer, has shown that water at intermediate depths (c1000m) off Central California in the late Pleistocene was far better equilibrated with the atmosphere than the present-day Pacific Intermediate Water.
doi:10.2172/15001984 fatcat:a2prgzevwneutjlnuouwjyblh4