STRONTIUM (87SR/86SR) AND LITHIUM (δ7LI) ISOTOPIC EVIDENCE FOR SHIFTS IN DEVONIAN WEATHERING PATTERNS: IMPLICATIONS FOR THE EXPANSION OF LAND PLANTS AND THE ONSET OF THE ACADIAN OROGENY

The Devonian marks a time of radical change that fundamentally altered our planet, culminating in one of the five largest mass extinctions. Other changes include the major radiation and diversification of land plants, as well as major mountain building events like the Acadian and Antler orogenies. These changes profoundly shifted weathering and sedimentation patterns, evidenced by greater mudrock abundances and an increase in the occurrence of black shale facies. The time resolution of these weathering proxies, however, is rather coarse (several Myr) and not continuous, so it remains unclear when the onset of these weathering shifts occurred.

In this study we used traditional (Sr) and new (Li) geochemical proxies on Devonian rocks to constrain the timing of weathering shifts. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) can be used to estimate changes in continental weathering rates and sources. The Li isotope (δ⁷Li) proxy can be used to help constrain the Sr record when measured from rocks that pass diagenetic screening tests for siliciclastic contamination (similarity of ⁸⁷Sr/⁸⁶Sr values to the Sr seawater curve, Mn/Sr < 3, Rb/Sr <1, and Mg/Ca <0.5). New ⁸⁷Sr/⁸⁶Sr data, which mostly match the seawater curve, decrease steadily from more radiogenic values of 0.7089 during the earliest Devonian to 0.7078 by the mid Eifelian (Middle Devonian), followed by a gradual increase to 0.7084 by the end Devonian. New Li data exhibit a similar trend to the Sr record where δ⁷Li values are positive during the earliest Devonian (+10‰), reach a relative low (-0.5‰) during the Middle Devonian, and increase slightly (to ~+4‰) by the end Devonian. One key difference, however, is that there is a rapid δ⁷Li increase to +18‰ during the Eifelian about when ⁸⁷Sr/⁸⁶Sr values begin to increase. Inflections in these isotopic trends may represent a shift to increased weathering of more radiogenic sources, possibly associated with the onset of the Acadian orogeny. Furthermore, the rapid increase in d⁷Li values is consistent with an increase in reverse weathering and clay formation, possibly caused by the expansion of land plants and higher soil formation rates as orogenesis initiated. Higher weathering rates occurring during the onset of the diversity loss challenges the notion that anoxia was the main cause of the Late Devonian mass extinction.