Enhanced continental weathering activity at the onset of the mid-Cenomanian Event (MCE)

Abstract

The emplacement of a Large Igneous Province (LIP) is implicated in the triggering of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2). Evidence for a similar initiation mechanism for the mid-Cenomanian Event (MCE) is unclear. In this study, a reconstruction of mid-Cenomanian seawater 187 Os/ 188 Os, the first for the Western Interior Seaway, tests the competing roles of LIP versus continental weathering activity in triggering the MCE. The absence of a prolonged unradiogenic Os isotope excursion (low 187 Os/ 188 Os) at the onset of the MCE interval argues against LIP involvement in the event's initiation. Rather, more radiogenic 187 Os/ 188 Os at the onset, that continues to rise to the middle of the MCE, indicates that the event was triggered by increased continental weathering. The combination of decreasing 187 Os/ 188 Os from the middle of the MCE onward, coincident with a 40 Ar/ 39 Ar age of 96.4 Ma of basalts from Ellesmere Island, Canada, is consistent with High Arctic LIP-related volcanic activity that may have contributed to the end of the MCE. These new data on the MCE thus indicate that LIP activity is not always the trigger for carbon cycle perturbation and associated climate change.

Oceanic Anoxic Event 2 triggered by Kerguelen volcanism

“…Tie point B is placed at the onset of a significant positive CIE in 13 C (organic) (2‰ at U1513 and 1.5‰ at U1516) and 13 C (carbonate) (1‰ at U1513 and 2‰ at U1516), and a concurrent spike in NGR data. Biostratigraphic markers indicate an age between 98 and 96 Ma, consistent with the MCE (96.5 Ma) 8 . In other Northern Hemisphere records, the MCE is defined by a positive CIE ~ 1‰ 71 , and we here contribute a Southern Hemisphere record of the MCE.…”

“…However, this is in contrast to records from the Maverick Basin (southern WIS) and Demerara Rise (proto-North Atlantic), where elevated Hg/TOC values occur mid-way through the CIE 22 . Despite an absence of low Os i values, indicating volcanism may not have been the primary driver 8 , 12 , 13 , 20 , our Hg records suggest volcanism may have had a role in triggering the MCE, although the source was likely distal to the Mentelle Basin.…”

“…It is often expressed in the sedimentary record as a short-lived negative δ 13 C carbon isotope excursion (CIE) 5 followed by a broad positive CIE in bulk organic matter ( ~5‰) and carbonate (2-3‰), although organic carbon isotopes do not always mirror carbonate trends 6 . The Mid-Cenomanian Event (MCE) is a smaller-scale, short-term anoxic event that occurred ~96.5 Ma, with a positive δ 13 C excursion ~1‰ 7,8 , and a shift in foraminiferal, radiolarian, and calcareous nannofossil assemblages 9,10 . Initiation of OAE2 warming is thought to have enhanced nutrient cycling and stimulated high productivity, explaining the deposition of organicrich material, the broad positive CIE, and the expansion of oxygen minimum zones (OMZs) due to enhanced decomposition within the water column 11 .…”