Atmospheric carbon dioxide as a driver for deglaciation during the Mi-1 event: new evidence from terrestrial Southern Hemisphere proxies
Abstract
Foulden Maar is an annually-resolved maar lake deposit dating from the Oligocene/Miocene boundary. The deposit, from the South Island of New Zealand, is the first high-resolution terrestrial record of the O/M boundary and the rapid deglaciation of Antarctica that occurred during the second half of the Mi-1 event. A ~180 m core from the centre of the lake bed comprises ~60 m of basal graded breccias, sands and muds overlain by ~120 m of diatomite punctuated by volcanogenic horizons. The basal siliciclastic sediments contain clasts of basalt and country rock and are interpreted as diatreme breccias coeval with the formation of the maar. The diatomite succession consists of mm-scale light-dark couplets and diatomaceous turbidites. Radiometric dates were obtained from basaltic clasts found at ~110 m depth (close to the base of the diatomite sucession) in a slump deposit of crater wall material. These give ages of 23.45 ± 0.25 Ma and 23.68 ± 0.36 Ma. A nearby basaltic dyke formed during the same episode of volcanism as the maar crater gives a date of 23.17 ± 0.17 Ma. A magnetic reversal occurs at ~106 m depth in the core, constraining the age of this point to 23.34 Ma (the base of chron C6Cn.3n) or 23.03 Ma (the base of chron C6Cn.2n). Spectral analysis of physical properties measurements of the diatomite section of the core reveals obliquity and precessional frequencies. An age model based on these frequencies shows that individual light-dark couplets of diatomite represent annual varves and that the normally magnetised section from ~106 m depth to the top of the core covers ~100,000 years. This rules out C6Cn.3n, which is only 50,000 years long, placing the base of the diatomite succession at the Oligocene-Miocene boundary and the peak of the Mi-1 event. We have collected stomatal index values from Litsea and Podocarpus leaves found in the succession. The Podocarpus values are calibrated using Podocarpus plants grown at various concentrations of carbon dioxide from 380 ppmv to 1500 ppmv. The Litsea values are calibrated using published SI values for various Litsea species of similar morphology and inferred ecology (the NLE approach). Our results show an atmospheric carbon dioxide level of ~400 ppmv at ~23.01 Ma (77 m depth below the top of the core), with concentration increasing to ~800-1200 ppmv at ~22.98 Ma (55 m) and dropping back to ~400-600 ppmv at ~22.9 Ma (0 m, the present-day surface outcrop). This short-lived, rapid increase in atmospheric carbon dioxide concentration coincides with the initiation of the deglaciation phase of the Mi-1 event and implies that CO2 was the driver of this deglaciation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMPP13A1806F
- Keywords:
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- 1610 GLOBAL CHANGE / Atmosphere;
- 3344 ATMOSPHERIC PROCESSES / Paleoclimatology;
- 4901 PALEOCEANOGRAPHY / Abrupt/rapid climate change;
- 4930 PALEOCEANOGRAPHY / Greenhouse gases