ABSTRACT

The global carbon cycle underwent a major transition at the end of the last ice age ~12,000 years ago ? atmospheric carbon dioxide levels rose abruptly at that time. This project will use naturally-occurring ?radiocarbon? (the radioactive carbon isotope 14C) to study how global carbon pools changed during that deglacial transition. The project will test previous radiocarbon evidence for a pulse of carbon release from seafloor volcanoes in the eastern Pacific Ocean at that time. That result challenges the idea that carbon release from the solid Earth is relatively slow and steady. Chemical and isotope measurements on fossil shells from deep-sea sediment cores will show whether the carbon pulse came in the form of acid or base. This will help pin down the source of the carbon. Carbon cycle models will be used to estimate the scale of these carbon pulses and their impact on ocean acidity and atmospheric carbon dioxide levels. Preliminary data and initial model results suggest that the carbon source was acid/base neutral. If so, then enormous quantities of carbon could have been released without causing strong ocean acidification or a strong atmospheric carbon dioxide rise. Carbon dioxide is a greenhouse gas and human activities are responsible for a large release of carbon dioxide today. Documenting the scale and nature of the natural carbon release at the end of the last ice age will help predict the environmental consequences of human carbon release. It will also help predict whether the human carbon release may be neutralized by natural processes.

The Gulf of California study site was selected because it contains: (1) a high-quality wood fragment-based chronology, (2) abundant and well-preserved benthic foraminifera for boron and carbon isotope analysis, (3) replicated evidence for regionally and temporally coherent 14C anomalies, and (4) known local sources of geologic carbon associated with the East Pacific Rise. The main thrust of the proposed work comes from d11B and B/Ca measurements to establish if there was any seawater carbonate chemistry and/or pH change associated with the 14C anomalies, but the proposed work also includes a wider set of complementary isotope geochemical measurements (14C/C, d13C) to enhance the value of the database as a constraint on the possible explanations for the regional 14C anomalies. The project also includes regional and global carbon cycle modeling to assimilate the multi-proxy constraints and quantitatively assess the implied effects on global seawater carbon chemistry and atmospheric CO2. This work will inform the active debate about the contribution of deglacial carbon release to deglacial CO2 rise. The working hypothesis is that significant pulses of geologic carbon releases explain the observed 14C anomalies but would not significantly contribute to CO2 change because that carbon came in neutralized bicarbonate ion form. Overall, the study will take on a significant gap in understanding natural carbon cycle change on human-relevant timescales by collaboratively integrating novel multi-proxy measurements with carbon cycle modeling. Both PIs are early career scientists with demonstrated domain expertise and track record.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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