Pulses of enhanced continental weathering associated with multiple Late Devonian climate perturbations: Evidence from osmium-isotope compositions

Abstract

Anomalously high rates of continental weathering have frequently been proposed as a key stimulus for the development of widespread marine anoxia during a number of Late Devonian environmental and biospheric crises, which included a major mass extinction during the Frasnian-Famennian transition (marked by the Upper and Lower Kellwasser horizons). Here, this model is investigated by presenting the first stratigraphic record of osmium-isotope trends (187 Os/ 188 Os) in upper Devonian strata from the Kowala Quarry (Holy Cross Mountains, Poland). Changes in reconstructed 187 Os/ 188 Os seawater values to more radiogenic compositions are documented at the base of both the Lower (~0.42 to ~0.83) and Upper (~0.31 to ~0.81) Kellwasser horizons characteristic of the Frasnian-Famennian transition, and additionally within upper Famennian shales that record a more minor environmental perturbation known as the Annulata Event (~0.20 to ~0.53). These shifts indicate the occurrence of extremely enhanced continental weathering rates at the onsets of the Kellwasser crises and during the later Annulata Event. The similarity of 187 Os/ 188 Os values in this study from Frasnian-Famennian boundary and lower Famennian strata (between 0.4-0.5) to those from North American stratigraphic equivalents suggests that the 187 Os/ 188 Os values record global trends. These findings support a causal relationship between increased continental weathering (and thus, nutrient supply to the marine shelf) and the environmental perturbations that occurred during numerous Late Devonian events, including both of the biospherically catastrophic Kellwasser crises as well as other, less severe, oceanic anoxic events.

Phosphorus-cycle disturbances during the Late Devonian anoxic events

“…However, unlike at Steinbruch Schmidt where non-Kellwasser strata record a continuously welloxygenated water column, several studies indicate that redox conditions at Kowala outside of the times of the Kellwasser events were much more variable, alternating from suboxic at times all the way up to intermittent episodes of euxinia (e.g., Bond and Zatoń, 2003;Bond et al, 2004;Marynowski et al, 2011). Increased levels of primary productivity and detrital input have also been reported from the UKW Horizon (Racki et al, 2002;Pujol et al, 2006), whilst shifts in reconstructed osmium-isotope compositions towards radiogenic values further support hypotheses that the Kellwasser horizons at Kowala were deposited during times of globally enhanced continental weathering (Percival et al, 2019).…”

“…The LKW Horizon is not clearly defined at Kowala. Percival et al (2019) interpreted a set of laminated organic-rich shales from 12 m below the F-F boundary that featured both organic and inorganic geochemical indications of oxygen-depleted conditions as the LKW strata, and this interpretation is followed for this study, but there is no evidence of a positive excursion in δ 13 C values in either bulk carbonate or organic material.…”

“…In contrast, the UKW Horizon is very well defined at Kowala, with euxinic conditions indicated by the appearance of finely laminated organic-rich micrites and shales, fine grained framboidal pyrite populations, and trace-metal content perturbations (Joachimski et al, 2001;Bond et al, 2004). A clear positive excursion in δ 13 C is also documented in both organic matter and bulk carbonates in the UKW Horizon (Joachimski et al, 2001;Percival et al 2019).…”

“…Alternatively, it has been proposed that an increase in continental weathering rates and associated runoff of terrigenous nutrients stimulated the enhanced primary productivity and subsequent marine anoxia (e.g., Wilder, 1994;Algeo and Scheckler, 1998;Averbuch et al, 2005). Increased terrestrial runoff associated with enhanced continental weathering during the Devonian crises has been documented by a number of proxies (e.g., strontium-isotopes, Chen et al, 2005; osmium-isotopes, Percival et al, 2019; silicon/aluminum (Si/Al), titanium/aluminium (Ti/Al), and zirconium/rubidium (Zr/Rb) ratios, Racki et al, 2002;Pujol et al, 2006;Riquier et al, 2006;Weiner et al, 2017;Paschall et al, 2019), with evidence of soil erosion and wildfires from organic-geochemistry studies further highlighting the contribution of terrestrial matter to the marine realm (Marynowski et al, 2012;Kaiho et al, 2013;Rimmer et al, 2015). Here, the distribution of nutrients in marine shelf basins during Late Devonian anoxic events, as well as their potential source, is further investigated, using phosphorus as a tracer of nutrient influx and recycling at a number of new records of Upper Devonian strata with a wide palaeogeographical coverage.…”

Osmium isotope and trace elements reveal melting of Chhota Shigri Glacier, western Himalaya, insensitive to anthropogenic emission residues

“…Total procedural blanks were 2.1± 0.02 and 0.1± 0.01 ppt for Re and Os, respectively, with an average 187 Os/ 188 Os value of 0.25± 0.03 (n = 2) for cryoconite and moraine analysis, and 2.3± 0.2 and 0.1± 0.02 ppt for Re and Os, respectively, with an average 187 Os/ 188 Os value of 0.20± 0.06 (n = 4) for coal and exhaust particulate analysis. In-house standard solution measurements yielded 185 Re/ 187 Re value of 0.59786± 0.00014 (1 SD, n = 7) for the Re solution 20 and a 187 Os/ 188 Os value of 0.16085± 0.00017 (1 SD, n = 6) for the Durham Romil Osmium Solution (DROsS), which are in agreement with those reported in the previous studies (Percival et al, 2019).…”