Weathering responses to rapid climate change: analysis of a high resolution osmium isotope record at the Paleocene-Eocene Thermal Maximum
Open Access
- Author:
- Wieczorek, Rebecca A.
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Matthew Scott Fantle, Thesis Advisor/Co-Advisor
Lee Kump, Thesis Advisor/Co-Advisor - Keywords:
- PETM
osmium
Spitsbergen
Arctic
weathering - Abstract:
- The Paleocene-Eocene Thermal Maximum (PETM, ~55.8 million years ago; Ma) was an abrupt ~170 thousand year (ky) global warming event in which a large amount of carbon was added to the atmosphere-ocean system over a few thousand years. According to our current understanding, such perturbations of the carbon cycle are regulated over geologic timescales by weathering processes, as atmospheric carbon is removed through the process of silicate weathering. Evidence of past weathering (“paleoweathering”) is determined through the use of isotope proxies; the marine osmium isotope record is one proxy commonly utilized for its ability to record geologically rapid events due to osmium’s short (10 ky) residence time in the ocean. Here I present an osmium isotope record (187Os/188Os) from Spitsbergen, Svalbard (core BH 9/05) spanning the PETM interval. The core is a marine grey-shale sequence with the PETM-defining carbon isotope excursion spanning ~50 m, making it ideal for high-resolution analysis. For this study, forty-four bulk rock samples were digested using Carius tube procedures and/or NiS fire assay bulk fusions and their rhenium (Re) and Os concentrations and Os isotopic compositions measured using inductively-coupled plasma mass spectrometry (ICP-MS). Bulk rock Os concentrations range from 111 to 207 pg/g while Re concentrations range from 0.7 to 13.1 ppb. Both Re and Os concentrations increase over the PETM interval, coincident with an interval of laminated shales and elevated pyrite content, suggesting regional euxinic conditions. 187Os/188Os ratios range from 0.3557 to 0.8238, with a definitive maximum ~6 ky after the onset of the carbon isotope excursion (CIE), concurrent with a large increase in the proportion of kaolinite. These increases in 187Os/188Os ratios and kaolinite content are interpreted as an increase in continental weathering through increased riverine Os input rather than lithologic source change. Modeling also supports this conclusion, though influence from a source change cannot be absolutely ruled out. Bulk elemental oxide concentrations were also measured in shales spanning the PETM in order to provide additional constraints on both timing of events and potential weathering sources. Bulk oxides are used to calculate the chemical index of alteration (CIA), a common weathering intensity index. CIA measurements indicate source material deposited during the onset of the PETM was intensely weathered. Additionally, this independent CIA record is nearly identical to the 187Os/188Os record, with coincident maxima and minima, supporting the hypothesis that variations in both proxies are linked to either a change in source or variation in the local environment. Though BH 9/05 reflects the shape of contemporaneous Os isotope records, the measured 187Os/188Os ratios in BH 9/05 are nearly double those of open ocean seawater (~0.3-0.45) before and after the PETM. This leads to the inference that BH 9/05 represents a restricted environment rather than the open ocean, most likely a basin environment with limited ocean contact and periodic episodes of anoxia. Modeling yields a global river increase of 40-50% while smaller rivers may have tripled their Os output over this interval. Overall, though the restricted basin setting of BH 9/05 may limit conclusions about global weathering patterns at the PETM, it simultaneously provides an important look at the paleo-Arctic response to rapid warming and may give further insight into climate change in the polar regions.