Abstract :
[en] To what extent are individual middle Miocene eccentricity-scale benthic foraminiferal
carbon isotope maxima (the so-called CM events) related to changes in marine export
productivity? Here we use benthic foraminiferal accumulation rates from three sites in the
Pacific and Southern Oceans and a geochemical box model to assess relationships between
benthic foraminiferal δ13C records, export productivity, and the global carbon cycle. Results
from Deep Sea Drilling Project Hole 588 and Ocean Drilling Program Site 747 show a
distinct productivity maximum during CM 6 at 13.8 Ma, the time of major expansion of ice
on Antarctica. Productivity maxima during other CM events are only recorded at
high-latitude Site 747. A set of numerical experiments tests whether changes in
foraminiferal δ13C records (CM events) and export productivity can be simulated solely by
sea level fluctuations and the associated changes in global weathering-deposition cycles, by
sea level fluctuations plus global climatic cooling, and by sea level fluctuations plus
invigorated ocean circulation. Consistent with data, the periodic forcing of sea level and
albedo (and associated weathering cycles) produces δ13C variations of the correct temporal
spacing, albeit with a reduced amplitude. A productivity response of the correct magnitude
is achieved by enhancing ocean circulation during cold periods. We suggest that the pacing
of middle Miocene δ13C fluctuations is associated with cyclical sea level variations. The
amplitude, however, is muted perhaps due to the competing effects of a time-lagged
response to sea level lowstands but an immediate response to invigorated ocean circulation
during cold phases.
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