Abstract
Abstract. The Paleocene–Eocene Thermal Maximum (PETM, ~ 56 Ma) was a ~ 200 kyr episode of global warming, associated with massive injections of 13C-depleted carbon into the ocean–atmosphere system. Although climate change during the PETM is relatively well constrained, effects on marine oxygen concentrations and nutrient cycling remain largely unclear. We identify the PETM in a sediment core from the US margin of the Gulf of Mexico. Biomarker-based paleotemperature proxies (methylation of branched tetraether–cyclization of branched tetraether (MBT–CBT) and TEX86) indicate that continental air and sea surface temperatures warmed from 27–29 to ~ 35 °C, although variations in the relative abundances of terrestrial and marine biomarkers may have influenced these estimates. Vegetation changes, as recorded from pollen assemblages, support this warming. The PETM is bracketed by two unconformities. It overlies Paleocene silt- and mudstones and is rich in angular (thus in situ produced; autochthonous) glauconite grains, which indicate sedimentary condensation. A drop in the relative abundance of terrestrial organic matter and changes in the dinoflagellate cyst assemblages suggest that rising sea level shifted the deposition of terrigenous material landward. This is consistent with previous findings of eustatic sea level rise during the PETM. Regionally, the attribution of the glauconite-rich unit to the PETM implicates the dating of a primate fossil, argued to represent the oldest North American specimen on record. The biomarker isorenieratene within the PETM indicates that euxinic photic zone conditions developed, likely seasonally, along the Gulf Coastal Plain. A global data compilation indicates that O2 concentrations dropped in all ocean basins in response to warming, hydrological change, and carbon cycle feedbacks. This culminated in (seasonal) anoxia along many continental margins, analogous to modern trends. Seafloor deoxygenation and widespread (seasonal) anoxia likely caused phosphorus regeneration from suboxic and anoxic sediments. We argue that this fueled shelf eutrophication, as widely recorded from microfossil studies, increasing organic carbon burial along many continental margins as a negative feedback to carbon input and global warming. If properly quantified with future work, the PETM offers the opportunity to assess the biogeochemical effects of enhanced phosphorus regeneration, as well as the timescales on which this feedback operates in view of modern and future ocean deoxygenation.
Highlights
The Paleocene–Eocene Thermal maximum (PETM; ∼ 56 Ma) is one of at least three geologically brief (< 200 kyr) global warming events, often referred to as “hyperthermals” (Thomas and Zachos, 2000), superimposed on a long-term late Paleocene–early Eocene warming trend (Zachos et al, 2008; Bijl et al, 2013; Frieling et al, 2014)
We present a multiproxy stratigraphic, paleoecologic, and paleoclimatologic study of late Paleocene and early Eocene sediments recovered in the Harrell Core, which was drilled by the Office of Geology of the Mississippi Department of Environmental Quality near Meridian, MS (NE, NW, NW, NW, Township 15N, Range 15E, Section 24, Lauderdale County; converted to latitude and longitude using www.earthpoint.us: 32◦15 04 N, 88◦43 14 W), and ∼ 9.7 km south-southwest of the Red Hot Truck Stop locality (Fig. 1)
Average values are −25.5 ‰ (1σ = 0.25), while above it to 118.6 mbs, values average −27.5 ‰ (1σ = 0.70), with minimum values of −28.6 ‰ (Fig. 3). The onset of this carbon isotope excursion (CIE) corresponds to a ∼ 10 cm thick sand-rich and highly micaceous interval. Because this CIE could theoretically reflect an increase in the relative abundance of marine organic matter over terrestrial organic matter (Sluijs and Dickens, 2012), we tested if it is present in specific biomarkers
Summary
The Paleocene–Eocene Thermal maximum (PETM; ∼ 56 Ma) is one of at least three geologically brief (< 200 kyr) global warming events, often referred to as “hyperthermals” (Thomas and Zachos, 2000), superimposed on a long-term late Paleocene–early Eocene warming trend (Zachos et al, 2008; Bijl et al, 2013; Frieling et al, 2014). One of the proposed forcing mechanisms for marine biotic change has been deoxygenation (Thomas, 2007) This is because several studies have presented evidence of a decrease in water column oxygen content in deep, intermediate and shallow settings, the full extent and consequences remain unclear (Speijer and Wagner, 2002; Sluijs et al, 2006; Chun et al, 2010; Dickson et al, 2012). The first identification of the PETM within the GCP and documentation of sea surface and continental warming has regional implications for stratigraphy and primate migration patterns These new data, combined with an extensive data compilation, suggest that ocean deoxygenation played a major role in nutrient cycling, biotic changes, and carbon cycle evolution during the PETM
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