Abstract

TOWARD AN ASTRONOMICALLY-TUNED HIGH-RESOLUTION BENTHIC ISOTOPE STRATIGRAPHY FOR THE PALEOCENE AND EARLY EOCENE

JAMES ZACHOS, UNIVERSITY OF CALIFORNIA, SANTA CRUZ EAR-0959117

ABSTRACT The late Paleocene to early Eocene (~58 to 52 Mya) represents one of the most climatically dynamic periods of Earth?s past. It is characterized by a gradual long warming trend that culminates in the warmest conditions of the last 90 m.y., the Early Eocene Climatic Optimum. Recent investigations, however, have found that the long-term warming trend was punctuated by several short-lived (transient) warming events (tens of thousands of years in duration), referred to as hyperthermals or Eocene thermal maxima. These events, which include the extreme Paleocene-Eocene Thermal Maximum (PETM, ~56 Mya), are thought to have a common origin, possibly involving orbital forcing amplified by carbon cycle feedbacks. Other proposed driving mechanisms, particularly for the PETM, include volcanic outgassing and/or the decomposition of marine methane hydrates. To assess whether the PETM and other thermal maxima were triggered by orbital or other forcing, we intend to establish the temporal relationship of these events to the long-term background variability (i.e., periodicity) in climate and the cabon cycle. To this end, utilizing a deep sea core recovered from the south Atlantic, we will construct the first high-resolution, astronomically-tuned benthic foraminiferal stable carbon and oxygen isotope time series for a ~5-6 m.y. segment of the upper Paleocene-lower Eocene. These and other records will allow us to establish the relative timing and phasing of the thermal maxima to excursions in the carbon cycle, thus providing constraints for quantitatively evaluating the nature of climate ? carbon cycle coupling during periods of warming. In addition to this application, the astronomically-tuned isotope time-series will serve as a standard isotope reference section by which all other Paleocene and early Eocene deep-sea sequences can be compared. In this capacity, it will be possible to use the carbon isotopes to correlate and calibrate segments of marine sedimentary sections to our tuned record, thereby significantly improving the accuracy of age models. This will facilitate transfer of the astronomical calibrations to sections with exceptional magnetostratigraphy, and thus the geomagnetic polarity time-scale.

Project Outcomes: The ability to resolve the true character of global scale climatic change over EarthÆs distant past requires the construction of high resolution stable carbon and oxygen isotope records in expanded marine sequences. Ideally, the resolution should be sufficient to capture cycles associated with variations in Earth's orbit. By capturing the highest frequency variability, such records provide the basic framework for both establishing the timing and pattern of changes in ocean temperature and the carbon cycle, and hence for correlating marine records from around the globe. The latter is essential for a variety of applications including building synoptic reconstructions of the general climatic conditions, such as sea surface temperature patterns, on a global scale for any given period. The results of this project provide the first such framework for a climatically critical period in EarthÆs past, the Paleocene-Eocene epochs, between 52 and 60 million years ago. Continental and ocean temperatures were much higher during these epochs, elevated primarily by higher levels of greenhouse gases. Our findings detail the scale of warming on long and short time scales. The latter includes both cyclical warming and rare excursions, or thermal maxima. All of the high frequency variability coincides with perturbations to ocean carbon chemistry, thus indicating amplification of climate change through feedbacks in the carbon cycle. The largest amplitude cycles appear to be occurring every 400 kyr, and were thus paced by well-known variations in earthÆs orbit. The infrequent transient warmings represent anomalous jumps into more extreme warm states. Our findings confirm that these were associated with strong positive feedbacks in the carbon cycle. In addition to the scientific contributions, the students and post-doctoral scholars who participated in this project received training in various aspects of laboratory and analytical methods, ranging from basic wet chemistry and micropaleontology, to advanced gas source mass spectrometery. Participants also received training in integrated cyclo- and chemostratigraphic techniques, and other advanced tools often employed in academic research and industry (exploration). The main findings of this project have been added to relevant lower and upper division undergraduate courses in Earth/Geologic sciences. Last Modified: 07/31/2013 Submitted by: James C Zachos