The Mid-Miocene "Monterey Event": A global productivity increase?
Zusammenfassung der Projektergebnisse
The aim of the project was to find the reasons for the long lasting positive carbon isotope excursion observed in benthic foraminifer records in the period from about 17 until 13.8 Ma (the Monterey Event). Our hypothesis was that it has been caused by enhanced sequestration of organic carbon in open ocean sediments and thus can be attributed to changes in marine primary productivity. To capture global trends, we investigated 6 DSDP/ODP drill sites from the Atlantic, Indian and Pacific Oceans bracketing that time interval. On sediments obtained from the core repositories, the two participating laboratories in Germany performed sedimentological-micropaleontological and geochemical studies; partners abroad determined stable isotope composition of benthic foraminifers (K. Billups, Delaware, USA) and organic carbon (D. Gröcke, Durham, UK). The evaluation of global changes in the carbon cycle and likely sources of the isotope signal observed in records was done with the aid of sophisticated geochemical and Isotopic box models at Liege University (L. Francois and V. Lefebvre). A publication about the three Atlantic sites that was published in February 2009 in Paleoceanography will be highlighted as a "Research Highlight" in the March issue of NATURE GEOSCIENCE: "Mid-Miocene Paleoproductivity in the Atlantic Ocean and Implications for the Global Carbon Cycle" by L. Diester-Haass, K. Billups, D.R. Gröcke, L. Francois, V. Lefebvre and K.-C. Emeis. Evaluation. The data for this first product of the project have been deposited with the World Data Center for Paleoclimatology and NOAA's National Climatic Data Center, USA. The summary of the results from the Atlantic sites presented as Research Highlight in the "Nature Geoscience" March 2009 Issue: A prolonged carbon isotope excursion during the Miocene (17.5 to 13.5 million years ago) period is attributed to enhanced burial of organic carbon. Observations from sites scattered throughout the Atlantic Ocean now rule out the ocean floor as the primary storage site. Liselotte Diester-Haass of the Universität des Saarlandes, Germany, and colleagues measured numerous proxies for past oceanic productivity from three marine sediment cores. They found evidence for occasional increases in local productivity throughout the Miocene, but no indications of a widespread spike in marine surface productivity associated with the isotopic excursion. Results from a numerical box model suggest that the excursion could instead have been driven by rising deposition of organic carbon in peat and wet lands. Despite the burial of so much organic carbon, the group points out that most proxy records indicate little to no accompanying decline in atmospheric carbon dioxide concentrations. Moreover, they show that the release of volcanic gases during the eruption of the Columbia River flood basalts could not have balanced the carbon sequestration. Where is then the missing source of Middle Miocene CO2?
