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Paleoclimate reconstructions suggest distinctive changes in the El Nino–Southern Oscillation (ENSO), the dominant mode of tropical climate variability, over the last glacial cycle and throughout the Holocene. However, to the best of our knowledge, no studies provide parallel reconstructions of warm (El Nino) and cold (La Nina) phase variability, thus precluding distinction between variations in ENSO activity and the mean state of the tropical Pacific. Here we provide the first such records, generated using molecular organic geochemical proxies in a sediment core from the Peru margin region. The opposing influences of El Nino and La Nina on coastal upwelling and phytoplankton community structure produce distinct sedimentary sterol records describing the evolution of each ENSO phase. We document changes in surface ocean productivity on the Peru margin over the past 16 ka that indicate enhanced La Nina–like conditions alone during deglaciation, but concomitant increases in both El Nino and La Nina activity during the past 2 ka. We propose that the preponderance of La Nina–like conditions prior to 11.5 ka reflects the influence of waning glacial boundary conditions. By contrast, warm and cold phase covariability during the late Holocene suggests centennial-scale changes in ENSO activity, rather than the Pacific mean state, driven by tropical insolation.

Records of vascular plant leaf wax δ 13 C and δ D were generated from terrestrial biomarkers preserved in Cariaco Basin marine sediments, and were found to parallel known regional millennial-scale climate changes between the late Glacial and Preboreal periods. The terrestrial biomarkers used consisted of C 24 –C 32 n -alkanoic acids, which originate from vascular plant leaf waxes. Differences in δ D between C 16 –C 18 and C 24 –C 30 n -alkanoic acid homologues suggest a marine source for the shorter chain lengths and a terrestrial source for the longer chains. Both the δ 13 C and δ D leaf wax records exhibited enrichment during the late Glacial and Younger Dryas and relative depletion during the Bolling/Allerod and Preboreal periods. δ D likely records the hydrogen isotopic composition of plant growth water, and thus may act as a proxy for local aridity. In agreement with Cariaco Basin sediment grey scale records, the δ D record suggests that the late Glacial and Younger Dryas were more arid than the Bolling/Allerod and Preboreal periods. n -Alkanoic acid δ 13 C, which is a proxy for C 3 versus C 4 plant type, indicates that C 3 plants predominated in this area of the tropics during warm and wet periods, such as the Bolling/Allerod and the early Holocene, and C 4 plants proliferated during cooler and more arid periods, such as the Glacial and Younger Dryas. The biomarker δ 13 C record agrees with pollen data from Cariaco Basin sediments, confirming that leaf wax compounds preserved in marine sediments can accurately record terrestrial vegetation changes.

[1] Paleoceanographic studies using benthic foraminiferal Cd as a nutrient tracer have provided a robust means of reconstructing glacial Atlantic Ocean water mass geometry, but a paucity of data from the South Atlantic above 1200 m has limited investigation of Antarctic Intermediate Water (AAIW) configuration and formation. A new Cd depth profile from Brazil margin sediments suggests that AAIW penetrated northward at 1100 m to at least 27°S in the glacial Atlantic. It exhibited substantially reduced δ13Cas values, confirming preliminary evidence that this AAIW was unique to the glacial Atlantic and that it formed differently than today, with less atmospheric contact.

[1] Semi-volatile organic compounds derived from burned and fresh vascular plant sources and preserved in high-altitude ice fields were detected and identified through use of recently developed analytical tools. Specifically, stir bar sorptive extraction and thermal desorption coupled with gas chromatography/time-of-flight mass spectrometry allowed measurement of multiple biomarkers in small sample volumes (≤30 ml). Among other compounds of interest, several diterpenoids, which suggest inputs from conifers and conifer burning, were identified in post-industrial era and older Holocene ice from the Sajama site in the Bolivian Andes, but not in a glacial period sample, consistent with aridity changes. Differences in biomarker assemblages between sites support the use of these compounds as regionally constrained recorders of vegetation and climate change. This study represents the first application of these analytical techniques to ice core research and the first indication that records of vegetation fires may be reconstructed from diterpenoids in ice.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2006

Identifying leads and lags between high- and low-latitude abrupt climate shifts is needed to understand where and how such events were triggered. Vascular plant biomarkers preserved in Cariaco basin sediments reveal rapid vegetation changes in northern South America during the last deglaciation, 15,000 to 10,000 years ago. Comparing the biomarker records to climate proxies from the same sediment core provides a precise measure of the relative timing of changes in different regions. Abrupt deglacial climate shifts in tropical and high-latitude North Atlantic regions were synchronous, whereas changes in tropical vegetation consistently lagged climate shifts by several decades.