title: Late Pliocene to early Pleistocene millennial-scale climate fluctuations and sea-level variability: A view from the tropical Pacific and the North Atlantic
creator: Jakob, Kim Alix
subject: ddc-550
subject: 550 Earth sciences
subject: ddc-560
subject: 560 Paleontology Paleozoology
description: In light of the need for high-fidelity, quantitative projections of future climate change, it is mandatory to understand climate variability at timescales that do justice to the tempo of anthropogenic climate change (i.e., sub-millennial). At the same time, information on the lower-frequency (i.e., orbital-scale) baseline characteristics of climate change underlying the short-term variability is required for a full mechanistic understanding. Such information can be obtained from time intervals of Earth’s history that comprise climatic boundary conditions as they are to be encountered in the near future.     In this context, this thesis aims to contribute toward accurately deciphering past climate fluctuations and sea-level variability on glacial-interglacial (G-IG) to (sub-)millennial timescales during past times of “near-modern” paleogeography and warmer-than-present climatic boundary conditions – the late Pliocene to early Pleistocene (~2.75–2.4 million years ago [Ma], Marine Isotope Stages [MIS] G6–95). This time period falls within the transition from a warmer Pliocene climate without larger-scale Northern Hemisphere ice sheets to a progressively cooler Pleistocene climate dominated by a stronger response of the climate/cryosphere system to orbital forcing, widely being termed “intensification of Northern Hemisphere Glaciation” (iNHG). The target interval includes MIS 100 through 96, three large-amplitude G-IG cycles (~1‰ in benthic δ18O) that represent the culmination of iNHG.     The first part of this thesis focuses on identifying the mechanisms that regulated G-IG primary productivity changes in the Eastern Equatorial Pacific (EEP) upwelling system, which constitutes an important component of the Earth’s atmospheric and marine carbon budget. For this purpose, high-resolution carbon isotopes from benthic and planktic foraminiferal calcite, and sand-accumulation rates have been investigated for the time interval between ~2.65 and 2.4 Ma (MIS G1–95) at Ocean Drilling Program (ODP) Site 849, which is located in the heart of the EEP upwelling regime. The presented records show increasing productivity levels for glacials from MIS 100 onward, with an onset during full glacial conditions and a maximum during glacial terminations. This productivity pattern is explained with enhanced nutrient delivery from the high southern latitudes during full glacial conditions in combination with superimposed intensified regional upwelling toward glacial terminations.    The second part of this thesis focuses on changes in equatorial Pacific surface-water structure across iNHG to elucidate whether these are related to El Niño- and La Niña-like climate dynamics. The strong inter-annual variability between El Niño and La Niña events in the modern tropical Pacific plays a major role in the global climate system. During the Pliocene, the Pacific warm pool was greatly expanded relative to today, implying a surface-ocean temperature field akin to modern El Niño events. The iNHG marks the transition from this warm Pliocene state toward modern conditions with strong upwelling in the EEP and stronger zonal temperature and primary productivity gradients. In this thesis, this transition is examined based on new, high-resolution planktic foraminiferal Mg/Ca- and δ18O-derived sea-surface temperature and salinity data for ODP Site 849 in the EEP for ~2.75–2.4 Ma (MIS G6–95). To investigate zonal gradients that are indicative for El Niño- and La Niña-like climate dynamics, these records are compared to similar records from the Western Equatorial Pacific. While no G-IG pattern is seen in the temperature gradient, obliquity-paced oscillations are documented in the salinity gradient. These oscillations originate from a response to a glacial southward and interglacial northward migration of the Intertropical Convergence Zone over the EEP rather than to fluctuations between El Niño- and La Niña-like climate states.    The final study of this thesis reconstructs high-resolution sea-level records for ODP Site 849 in the EEP and Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic covering ~2.65–2.4 Ma (MIS G1–95) based on benthic foraminiferal δ18O and Mg/Ca data. New sea-level lowstand estimates for MIS 100–96 show substantially lower values for Site 849 than for Site U1313 (peak values of ~55–85 m versus ~110–130 m below present), while interglacial highstand estimates are consistent (~0–30 m above present). In comparison to previous estimates, values for Site U1313 appear to be overestimated, most likely due to uncertainties induced by changing deep-water masses at that site. Nevertheless, the sea-level evolution for MIS 100 and 96 emerges as strikingly reminiscent of late Pleistocene glacials (asymmetric “sawtooth” structure, rapid termination, final sea-level drop before the termination). This suggests that similar mechanisms controlled early and late Pleistocene     G-IG cycles. Remarkably, the generated data from EEP Site 849 and North Atlantic Site U1313 imply a ~50–100 m sea-level rise into early Pleistocene interglacials. This would require complete melting of Greenland and West Antarctic Ice Sheets and a partial retreat of the “stable” East Antarctic Ice Sheet, hinting at its future vulnerability.
date: 2017
type: Dissertation
type: info:eu-repo/semantics/doctoralThesis
type: NonPeerReviewed
format: application/pdf
identifier: https://archiv.ub.uni-heidelberg.de/volltextserverhttps://archiv.ub.uni-heidelberg.de/volltextserver/22942/1/DissKJ_10mb.pdf
identifier: DOI:10.11588/heidok.00022942
identifier: urn:nbn:de:bsz:16-heidok-229425
identifier:   Jakob, Kim Alix  (2017) Late Pliocene to early Pleistocene millennial-scale climate fluctuations and sea-level variability: A view from the tropical Pacific and the North Atlantic.  [Dissertation]     
relation: https://archiv.ub.uni-heidelberg.de/volltextserver/22942/
rights: info:eu-repo/semantics/openAccess
rights: http://archiv.ub.uni-heidelberg.de/volltextserver/help/license_urhg.html
language: eng