The hydrologic cycle and associated terrestrial carbon cycle feedbacks during past hothouse climates

Abstract

Anthropogenic greenhouse gas (GHG) emissions continue to propel Earth towards a ‘hothouse’ world, with profound ramifications upon global biogeochemical cycles. Yet, the response of complex biogeochemical interactions remain highly uncertain, impeding accurate predictions of future climate change. The development of novel lipid biomarker-based proxies have created the opportunity to characterise the hydrologic cycle and associated terrestrial carbon cycle feedbacks (e.g., rock organic carbon oxidation and methane cycling) in the geologic record. This thesis explores the hothouse worlds (i.e. Paleocene and Eocene; ~66–34 Ma) and a hyperthermal event (i.e. Paleocene-Eocene Thermal Maximum; PETM; ~56 Ma) of the past, as they provide the closest analogue for a high emission/low mitigation climate scenario. Leaf wax biomarkers indicate that the tropics were characterised by an extremely variable hydrologic cycle during the latest Paleocene (~57–56 Ma). Such conditions may have had far-reaching consequences for rock organic carbon oxidation and methane cycling. To investigate the impact of an intensified hydrologic cycle, new biomarker-based records are established to reconstruct the mobilisation of ‘petrogenic’ organic carbon (OCpetro) and the methane cycle during the PETM. The data reveals widespread delivery of OCpetro to the oceans, which is attributed to an increase in extreme precipitation events. Raman spectroscopy is then used to evaluate whether the PETM also coincided with enhanced oxidation of OCpetro. Evidence suggests that there was a high OCpetro oxidation efficiency in the mid-latitudes, although this appears spatially variable. To reconstruct the methane cycle, a diverse range of bacteriohopanepolyols (BHPs) are identified in a PETM-aged lignite deposit, and specific BHPs show promise as a proxy for methanotrophy in the geologic past. Taken together, there are signs of an increase in both OCpetro and methane oxidation during the PETM. Overall, this implicates that these positive feedback mechanisms will likely be sensitive to transient warming in the future, and should thus be incorporated in the newest generation of climate models.

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Identifiers

ORCID for Emily H. Hollingsworth: orcid.org/0000-0002-7067-6837

ORCID for Gordon Inglis: orcid.org/0000-0002-0032-4668

ORCID for Gavin Foster: orcid.org/0000-0003-3688-9668

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