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John, Eleanor; Anand, Pallavi and James, Rachael
(2010).
Abstract
Many lines of evidence, from both marine and terrestrial records, suggest that there have been dramatic changes in the Earth’s climate system over the Cenozoic. The mechanisms that are responsible for these changes are less well established, but there is growing evidence that amongst the primary controls is the concentrations of atmospheric carbon dioxide, the principal greenhouse gas.
On long timescales (>105 yr), levels of atmospheric CO2 are controlled by the balance between CO2 input from mantle degassing, and CO2 removal by weathering of continental silicates rocks. For this reason, information on past levels of silicate weathering rates is essential for understanding the changes in Cenozoic climate. To this end, temporal records of Li/Ca and 7Li/6Li ratios measured in foraminifera are potentially useful, as long as seafloor spreading rates and changes in marine Ca concentrations can been accounted for. Foraminiferal 143Nd/144Nd ratios can provide additional information about the source of the weathering flux and patterns of oceanic circulation. The results of an ongoing study by Anand et al. (this meeting) examining weathering processes over the last 15 Ma show a two step increase in d7Li values from 15 Ma to present. The first step increase in δ7Li is observed between 13.7 and 8.7 Ma (from +18 to +30 ‰) and the second step increase was observed around ~4 Ma to present (d7Li values increased from +30 to +32 ‰). Published data for the last 18 Ma has shown a similar pattern but the magnitude of change was smaller. The eNd values in the study of Anand et al. follow a similar pattern. Here, we present preliminary data from a study investigating changes in Li/Ca, d7Li and eNd between 40 and 20 Ma thus extending the record.
Monospecific samples of mixed layer foraminifera were picked from deep sea core samples taken from the Ceara Rise (western equatorial Atlantic; ODP Site 925) and analysed for their trace element content (including Li/Ca and Mg/Ca) using ICP-MS, and their Li and Nd isotopic composition using MC-ICP-MS. Our preliminary data (8 samples) show an overall rise in Li/Ca from 8.0 to 10.3 μmol/mol between ~26 and 20 Ma (latest Oligocene – earliest Miocene), with a drop to 6.2 μmol/mol at around 23 Ma, albeit based on data from one sample. This suggests a slight increase in silicate weathering flux over this interval and a possible decrease at around the time of the Mi-1 event. Mg/Ca ratios from the same interval suggest mixed layer seawater temperatures of 19.4-22.5 ºC although no overall trend was observed. d7Li values, which gave excellent reproducibility on repeat measurements, from ~23-20 Ma (earliest Miocene) lie in the range 20.1-23.9 ‰ with a single sample yielding a value of 17.3 ‰. These values are the lowest reported to date from the Cenozoic (modern values = +32 ‰) and suggest that the low values measured by Anand et al. (this meeting) at around 13.7 Ma may have prevailed for several Ma. Further investigation will ascertain whether these low values are a result of diagenetic processes or are a primary seawater signal. If the latter is true, then these results suggest that chemical weathering was intense in the early Miocene. Additional Li data, as well as new Nd isotope data, will be presented at the meeting along with information about isotopic method refinement for high precision measurements.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)
