Ocean acidification and the δ15N record of Paleozoic epeiric seas
Abstract
In addition to its role as a primary driver of global climate, atmospheric CO2 influences the pH of seawater which is an important factor in mediating biogeochemical cycles. Variations in the pH of seawater on geological timescales have been correlated with broad impacts on marine ecosystems and biogeochemical processes including evolutionary turnover and mass extinction. Atmospheric CO2 declined dramatically during the mid-Paleozoic, coincident with the emergence of terrestrial forests and concomitant development of a substantial soil carbon reservoir and increased silicate weathering. Global greenhouse conditions that prevailed at the Late Devonian Frasnian/Famennian boundary gave way to temperate latitude glaciation at the end of the Famennian. In a recent review of icehouse-greenhouse variations in marine nitrogen biogeochemistry through the Phanerozoic (Algeo et al. 2014), the authors observed a strong correlation between sediment δ15N and first order climate cycles with a trend toward lower values during greenhouse periods and higher values during icehouse periods. Based upon modeling results, the shift in sediment δ15N was ascribed to a change in the locus of denitrification from sediments in warm climates to the water column during cooler periods driven primarily by eustatic sea level change as glacial ice mass waxed and waned. Sediment δ15N is a useful proxy for interpreting N biogeochemistry in marine systems because it provides an integrated record of the microbially-mediated redox reactions that led to that δ15N value. We propose that the elevated CO2 that drove the greenhouse climate in the early Famennian also resulted in the acidification of seawater that precluded nitrification, yielding an ammonium-dominated surface ocean and low sediment δ15N. As O2 climbed and seawater pH responded to diminished CO2, we propose that nitrification rates increased resulting in a nitrate-dominated system and sediment δ15N values that approach modern values. In support of our argument, we present stable isotope, redox, and compositional data from a core that spans the transition from the high CO2 greenhouse climate near the F/F boundary to the lower CO2 climate in the latest Devonian.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.B21A1952T
- Keywords:
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- 0424 Biosignatures and proxies;
- BIOGEOSCIENCES;
- 0454 Isotopic composition and chemistry;
- BIOGEOSCIENCES;
- 0465 Microbiology: ecology;
- physiology and genomics;
- BIOGEOSCIENCES;
- 1099 General or miscellaneous;
- GEOCHEMISTRY