Comparison of Early Jurassic 187Os/188Os i records

Other marine 187Os/188Os i records from the Lower Jurassic (Hettangian through Toarcian stages) generally show unradiogenic values20, 23, 29, 30. These are likely related to relatively elevated inputs of unradiogenic osmium from the weathering of the Central Atlantic Magmatic Province (CAMP) and the alteration of juvenile oceanic lithosphere or direct injection of mantle-derived osmium from initial opening of the North Atlantic31. The Upper Pliensbachian portion of our record from northeastern Panthalassa has broadly similar values to those observed in the European epicontinental sea20, 23, 29, which suggests they are representative of the global 187Os/188Os sw values, and indicative of a well-mixed Early Jurassic ocean. Further, the East Tributary 187Os/188Os i record shows a similar pattern to the other available records during the interval that contains the T-OAE20, 23. All the sites record an excursion to higher 187Os/188Os i values that follow the falling limb of the Toarcian negative CIE. This trend is followed by a return to lower 187Os/188Os i values after the rising limb of the negative CIE. However, in all cases 187Os/188Os i declines to values slightly higher than those observed before the excursion.

While all the 187Os/188Os i records display a similar overall pattern, their 187Os/188Os i values differ. The Yorkshire and East Tributary datasets have similar 187Os/188Os i values before and after the T-OAE (~0.3 and ~0.4, respectively); however, the Yorkshire dataset shows an excursion to significantly more radiogenic values (187Os/188Os i ≈ 1) during the T-OAE20 (Fig. 2). The Mochras data show higher 187Os/188Os i values just before the T-OAE CIE (~0.4), which increase to an acme of 0.8 during the T-OAE, and decrease to ~0.3 after the event23 (Fig. 2). While the absolute 187Os/188Os i values differ between the sites, the magnitude of the excursions at East Tributary and Mochras are similar at 0.4, and are almost half the magnitude observed at Yorkshire (0.7).

The differences observed between the 187Os/188Os i records at East Tributary, Mochras, and Yorkshire suggest there were regional differences in 187Os/188Os sw during the studied interval. These differences likely represent local processes such as differing degrees of hydrographic restriction from the open ocean and the amounts of local runoff and its 187Os/188Os composition. However, the similarity in the magnitude of the excursions recorded at East Tributary and Mochras suggest this likely represents the global record of change during the T-OAE. This observation, coupled with the more extreme 187Os/188Os i excursion record at Yorkshire, supports the suggestion that the Yorkshire 187Os/188Os sw record was influenced by a local riverine input of radiogenic osmium during the T-OAE21, and the East Tributary and Mochras records are more representative of global osmium seawater chemistry

With these observations in mind, we advocate, when possible, analyzing osmium isotope records from coeval stratigraphic successions deposited in different sedimentary and ocean basins18, 24,25,26 before attempting to interpret them as a global signal. This methodology is especially important regarding palaeoceanographic studies on intervals older than the Cretaceous since the preserved records are predominantly from continental margin and epicontinental successions, where geochemical signatures have a greater potential to be modified by local processes.

Quantifying the Early Jurassic marine osmium cycle

To gain a more quantitative measure of the changes in the marine osmium cycle during the Toarcian we employed a numerical box model that simulates the osmium inventory of the ocean and its isotopic composition (see Supplemental Information). Specifically, we test whether the osmium isotope excursion associated with the T-OAE (~300–500 kyr in duration)31, 32 can be reproduced by a transient increase in the weathering input of radiogenic osmium to the ocean. We also explored other situations that may have potentially driven the observed T-OAE osmium isotope record, but are likely implausible, such as decreasing the input flux of mantle-derived osmium to zero (see Table 1 for values explored and Supplemental Information for a discussion of these cases). Overall, the numerical model results show that the osmium isotope excursion can be reproduced by a transient three- to six-fold increase in the input of continental-derived osmium to the oceans over 100 to 200 kyr31, 32 (Fig. 4; more details of the modelling results including sensitivity tests can be found in the Supplemental Information).

Table 1 Range of parameters explored modelling the osmium isotope excursion associated with the Toarcian Oceanic Anoxic Event in the East Tributary and Yorkshire sections. Full size table

Figure 4 Examples of the modelled osmium isotopic composition of the ocean over the T-OAE. (A) For this model run, the osmium isotopic composition of the continental input was increased to 2.0 and the flux of osmium from continents was increased two-fold (475.3 mol/yr) during the Toarcian OAE. This resulted in the seawater osmium isotope values to increase to 0.44, which does not reproduce the observed osmium isotope excursion observed at East Tributary. (B) Model run where the osmium isotopic composition and flux of the continental input of osmium was increased to 2.0 by ~3.4x respectively. This model run reproduced the osmium isotope excursion. (C) The osmium isotope composition of the continental input of osmium was kept at 1.4 during the Toarcian OAE, but the flux of osmium from continents was increased by ~6.3x to reproduce the osmium isotope excursion. Full size image

Changes in the 187Os/188Os cont to more radiogenic values through the differential weathering of lithologies such as shales and cratonic rocks33,34,35 could have played a role in the T-OAE osmium isotope record. We investigated the potential effect this change would have on the osmium budget during the event by running simulations where we elevated 187Os/188Os cont from 1.4 to 2 (see Supplemental Information for a discussion of the choice of the maximum 187Os/188Os cont value). In these simulations, a nearly three-fold increase of the input of continental-derived osmium to the oceans was still necessary to reproduce the excursion (Fig. 4), regardless of timescale used, and solely increasing 187Os/188Os cont to reasonable values cannot reproduce the observed excursion (see Supplemental Information). Given the plausible proposition of the changing composition of the continental weathering flux, we conservatively suggest that T-OAE weathering rates increased by as much as three-fold.

A potential source of radiogenic, continentally derived osmium was the remnants of the Central Pangaean Mountains, a Himalayan-scale mountain belt in eastern North America and northwestern Africa. This mountain belt was positioned at tropical and subtropical latitudes in the Early Jurassic (Fig. 1). The rifting of Pangaea during the Late Triassic and Early Jurassic would have exposed the core of the mountain range leaving this material open to weathering or erosion. General circulation models predict large increases in the air temperature and runoff during the T-OAE in the geographic region that contained these mountains36. These regional climatic changes would have facilitated enhanced chemical weathering, and makes this mountain belt a plausible source of the enhanced input of osmium to the oceans advocated here.

The weathering of organic-rich rocks and sediments would be another plausible way to raise the isotopic composition of the continental weathering flux, but also results in a net release of CO 2 to the atmosphere37. However, enhanced continental runoff would also have increased nutrient delivery and stimulated primary productivity in aquatic environments leading to increased hypoxia, anoxia, and potentially euxinia5. Elevated burial of organic matter in these environments would have sequestered much more atmospheric CO 2 than that associated with any black shale weathering, which we suggest represent only a fraction of the continental materials that were predominantly weathered during the event.

Differences in the osmium isotope response between OAE events

A striking feature of the 187Os/188Os records during the Mesozoic OAEs is the directionality of their excursions. The T-OAE records show a positive 187Os/188Os excursion, whereas the onset of the Cretaceous OAE 1a and OAE 2 both display negative excursions. The difference in the 187Os/188Os response to these events most likely lies in the environment where the LIPs were emplaced. The Cretaceous events are associated with subaqueous emplacements of the Ontong Java Plateau (OAE 1a) and the Caribbean and High Arctic LIPs (OAE 2). Emplacement of these LIPs would have supplied large amounts of unradiogenic, mantle-derived osmium directly into the oceans from weathering of basalts on the seafloor, resulting in osmium isotope excursions to nonradiogenic values25, 38,39,40.

The T-OAE, on the other hand, is associated with a subaerial emplacement of the Karoo-Ferrar LIP at high latitudes (Fig. 1), where the semi-arid climate would have made the relative weathering potential of this material low. In contrast to the younger OAEs, the Toarcian 187Os/188Os i records reflect enhancement of the weathering of continental materials facilitated by the injection of greenhouse gases into the atmosphere and subsequent climate changes. Notably, delivery of osmium from the Karoo-Ferrar LIP would have also been delayed, as compared to the Cretaceous LIPs. However, if weathering of the Karoo-Ferrar LIP was a significant source of osmium to the oceans during the T-OAE, then its lower 187Os/188Os compositions41,42,43,44,45 would necessitate an even greater contribution of continental material to generate the observed 187Os/188Os i excursion.

Implications and Conclusions

Based on the osmium isotope records and our modelling results, the transient increase in continental weathering rates during the T-OAE may be one of the largest observed during the Phanerozoic. Chemical weathering rates are also suggested to have significantly increased across the Permian-Triassic boundary46, Triassic-Jurassic boundary47, 48, and the Paleocene-Eocene Thermal Maximum49, all of which are associated with intervals of global warming, environmental deterioration, and extinction events50. The rapid response of the osmium isotope system during the T-OAE, as well as during other OAEs38,39,40, indicates that chemical weathering feedbacks may respond to episodes of rapid climatic warming on short timescales (103–106 years) and lead to a net drawdown of atmospheric CO 2 5. Enhanced continental runoff would also have increased nutrient delivery and stimulated primary productivity in nearshore environments, leading to increased marine hypoxia, anoxia, and potentially euxinia5. CO 2 would also have been sequestered through the deposition of organic-rich sediments in marine and lacustrine settings5, 6, 51.