Response of the Cr isotope proxy to Cretaceous Ocean Anoxic Event 2 in a pelagic carbonate succession from the Western Interior Seaway

摘要

Chromium offers a redox sensitive isotopic proxy with potential for tracing past oxygen levels in the oceans. We explore this potential in a pelagic succession of marine carbonate sediment deposited during Cretaceous Ocean Anoxic Event 2 (OAE 2) in the Western Interior Seaway, Colorado (WIS), using the USGS Portland #1 Core. Reduction of Cr(VI) causes light isotopes of Cr to preferentially partition into Cr(III). Because Cr(VI) is the thermodynamically favoured species in oxygenated seawater, and Cr(III) is relatively insoluble under the same conditions, increased removal of Cr(III) into anoxic marine sediment during ocean anoxic events should cause positive shifts in seawater delta Cr-53 values. This assumes that isotopic fractionation associated with all Cr removal fluxes from the oceans was constant during OAE 2 and that there was no change in Cr input fluxes to the oceans. Here, we report findings that counter this prediction, namely evidence for a negative shift in seawater delta Cr-53 during OAE 2 of similar to 1.1% in the WIS. The magnitude of the excursion depends on the speciation of Cr removed in this setting over the duration of OAE 2, as well as the fractionation factor accompanying the removal flux of chromate into carbonate sediment, both of which are uncertain. The delta Cr-53 excursion reaches values as low similar to 0.09% during OAE 2, which is indistinguishable from the high-temperature igneous rock baseline value of -0.124 +/- 0.101% (2 sigma). Moreover, the minima in the delta Cr-53 profile coincides with a peak in Cr concentrations in the study core, as well as anomalous enrichments of other trace metals of basaltic affinity. Building on previous studies linking OAE 2 to a massive episode of submarine volcanic activity, we attribute the decrease in carbonate delta Cr-53 values during OAE 2 to the expansion of a hydrothermal superplume created during eruptions of the Caribbean Large Igneous Province (LIP). This metal laden plume, enriched in Cr(III) and probably anoxic, moved from deep waters of the eruption site in the eastern Pacific into deep waters of the proto-North Atlantic through an oceanic gateway in the Central Americas. Once inside, anoxic metal-rich waters upwelled against the surrounding continental margins, where isotopically unfractionated Cr(III), whose solubility was improved by organic complexation and/or oxidation to Cr(VI), mixed with the isotopically heavy pool of oceanic chromate in surface waters above the redoxcline. Application of a simple mixing equation shows that hydrothermal Cr contributed >90% of the authigenic Cr in the trace metal enriched interval of the study core. The increased flux of hydrothermal Cr was large enough to mask the expected isotopic response of the ocean Cr cycle to increasing anoxia. We consider this interpretation to be robust, in that it is not substantially affected by uncertainty in the speciation of hydrothermal Cr, nor uncertainty in the fractionation factor (Delta(CaCO3)(sw)) associated with chromate removal into marine carbonate sediment, which we measured in the modern Caribbean Sea to be -0.46 +/- 0.14% (2 sigma). (C) 2016 Elsevier Ltd. All rights reserved.