Dynamic of the Si cycle in the eastern Equatorial Pacific during the Miocene

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Author(s): Chetelat, Benjamin; Rickaby, R. E.; Halliday, Alex N.
Author Affiliation(s): Primary:
University of Oxford, Oxford, United Kingdom
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Today, the Eastern Equatorial Pacific (EEP) is a large region of primary productivity and opal production by diatoms. But this region also has a net CO2 efflux to the atmosphere due to the intensity of upwelling. As a result, competition between productivity related to external inputs of nutrients and production due to the local recycling of nutrients is thought to control the flux of carbon dioxide between the atmosphere and the ocean (Loubere, 2000). Thus, the degree of silica utilisation by diatoms in this area can affect the drawdown of atmospheric CO2 and illustrates the tight coupling between the Si and C cycles. The Miocene is a period of important climatic changes when the East Antarctic Ice Sheet became permanently established, followed by growth of the West Antarcic Ice Sheet and also a period characterized by the radiation of C4 grass on land which may have impacted the global Si cycle. We report here a 12 Myr record of diatom Si isotopes from site ODP850 located in the EEP. This site was previously characterized for its opal accumulation rates by Farrell et al. (1995) and the record shows two peaks in accumulation between 4.5-7 Myr and between 9.5-11 Myr. Our data from the Miocene all fall within the range of isotopic values reported for the last 35 kyr by Pichevin et al. (2009) from another site of the EEP. This suggests that the δ30Si of the ocean has not changed significantly over the last 12 Myrs such that there has been no drastic variation in the ratio of inputs to outputs of silica. Assuming constancy in the isotopic composition of upwelled waters, we can then interpret our δ30Si opal record in terms of silicic acid utilization and in comparison with associated opal accumulation rates, we bring new insights into the dynamics of the Si cycle for the EEP. The most recent 4.5 Myr are characterized by low opal accumulation rates and relatively high isotopic values compared with the period between 4.5 Myr and 7.5 Myr. This relationship is most likely a consequence of changing nutrient supply to the surface ocean, either due to upwelling intensity, or elevated silica concentrations in those upwelled waters: intense nutrient supply favoring the deposition of opal but resulting in lower silicic utilization and thus low opal Si isotopic compositions. By contrast, the period between 7.5 Myr and 12 Myr shows an increase of the Si isotopic composition with the opal accumulation rate suggesting that for this period diatom productivity is associated with a greater silicic acid utilisation. This illustrates the different dynamics of the Si cycle in the EEP between these two times period, with contrasting influence on atmospheric carbon dioxide. We will consider the consequences of opal preservation on this sedimentary record. In addition, we will also consider a novel record from the Southern Ocean that may allow us to refine our understanding of the Si cycle during the Miocene. Loubere (2000), Nature. Farrell et al. (1995), Proceedings of the Ocean Drilling Program, Scientific Results. Pichevin et al. (2009), Nature.
Year of Publication: 2011
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Algae; Cenozoic; Diatoms; East Pacific; Equatorial Pacific; Isotope ratios; Isotopes; Leg 138; Marine environment; Microfossils; Miocene; Neogene; North Pacific; Northeast Pacific; ODP Site 850; Ocean Drilling Program; Pacific Ocean; Paleo-oceanography; Paleoenvironment; Si-30/Si-28; Silicon; Silicon cycle; Stable isotopes; Tertiary
Coordinates: N011749 N011751 W1103116 W1103118
Record ID: 2018028847
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