Jacobson, A.
(Ed.)
Ocean anoxic events (OAE) are characterized by increased organic content of marine sediment on a global scale with
accompanying positive excursions in sedimentary organic and inorganic d 13C values. To sustain the increased C exports
and burial required to explain the C isotope excursion, increased supplies of nutrients to the oceans are often invoked during
ocean anoxic events. The potential source of nutrients in these events is investigated in this study for Oceanic Anoxic Event 2,
which spans the Cenomanian-Turonian boundary. Massive eruptions of one or more Large Igneous Provinces (LIPs) are the
proposed trigger for OAE 2. The global warming associated with volcanogenic loading of carbon dioxide to the atmosphere
has been associated with increased continental weathering rates during OAE 2, and by extension, enhanced nutrient supplies
to the oceans. Seawater interactions with hot basalts at LIP eruption sites can further deliver ferrous iron and other reduced
metals to seawater that can stimulate increased productivity in surface waters and increased oxygen demand in deep waters.
The relative importance of continental and submarine weathering drivers of expanding ocean anoxia during OAE 2 are difficult
to disentangle. In this paper, a box model of the marine Sr cycle is used to constrain the timing and relative magnitudes
of changes in the continental weathering and hydrothermal Sr fluxes to the oceans during OAE 2 using a new high-resolution
record of seawater 87Sr/86Sr ratios preserved in a marl-limestone succession from the Iona-1 core collected from the Eagle
Ford Formation in Texas. The results show that seawater 87Sr/86Sr ratios change synchronously with Os isotope evidence
for the onset of massive LIP volcanism 60 kyr before the positive C isotope excursion that traditionally marks the onset
of OAE 2. The higher temporal resolution of the seawater Sr isotope record presented in this study warrants a detailed quantitative
analysis of the changes in continental weathering and hydrothermal Sr inputs to the oceans during OAE 2. Using an
ocean Sr box model, it is found that increasing the continental weathering Sr flux by 1.8-times captures the change in seawater
87Sr/86Sr recorded in the Iona-1 core. The increase in the continental weathering flux is smaller than the threefold
increase estimated by studies of seawater Ca isotope changes during OAE 2, suggesting that hydrothermal forcing may have
played a larger role in the development of ocean anoxic events than previously considered.
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