The Early Toarcian oceanic anoxic event: Paleoenvironmental and paleoclimatic change across the Alpine Tethys (Switzerland)

Abstract

Paleoenvironmental and paleoclimatic change associated with the Toarcian oceanic anoxic event (T-OAE) was evaluated in five successions located in Switzerland. They represent different paleogeographic settings across the Alpine Tethys: the northern shelf (Gipf, Riniken and Rietheim), the Sub-Briançonnais basin (Creux de l'Ours), and the Lombardian basin (Breggia). The multi-proxy approach chosen (whole-rock and clay mineralogy, phosphorus, major and trace elements) shows that local environmental conditions modulated the response to the T-OAE across the Alpine Tethys. On the northern shelf and in the Sub-Briançonnais basin, high kaolinite contents and detrital proxies (detrital index, Ti, Zr, Si) in the T-OAE interval suggest a change towards a warmer and more humid climate coupled with an increase in the chemical weathering rates. In contrast, low kaolinite content in the Lombardian basin is likely related to a more arid climate along the southern Tethys margin and/or to a deeper and more distal setting. Redox-sensitive trace-element (V, Mo, Cu, Ni) enrichments in the T-OAE intervals reveal that dysoxic to anoxic conditions developed on the northern shelf, whereas reducing conditions were less severe in the Sub-Briançonnais Global and Planetary Change 162 (2018) 53-68 2 basin. In the Lombardian basin well-oxygenated bottom water conditions prevailed. Phosphorus (P) speciation analysis was performed at Riniken and Creux de l'Ours. This is the first report of P speciation data for T-OAE sections, clearly suggesting that high P contents during this time interval are mainly linked to the presence of an authigenic phases and fish remains. The development of oxygen-depleted conditions during the T-OAE seems to have promoted the release of the organic-bound P back into the water column, thereby further sustaining primary productivity in a positive feedback loop.

Phosphogenesis during the Cenozoic transition from greenhouse to icehouse conditions: Upper Oligocene to lower Miocene siliceous, phosphate, and organic‐rich sediments near La Purísima, Baja California Sur, Mexico

“…Molybdenum contents are not correlated with any of the other parameters considered here, except for a very slight correlation with TOC. Its enrichment is higher than those measured in Toarcian dysoxic sediments, is comparable to those in recent upwelling centres, but falls below those in sapropels and sediments related to the C-T OAE (Brumsack, 2006;Fantasia et al, 2018). As such, it indicates dysoxic to anoxic, but not yet euxinic conditions, with the possible exceptions of sediments around 10 m, 40 m, 55 m, and 75 m in the measured section ( Figure 12).…”

“…This indicates that V is not preferentially adsorbed onto organic matter or associated with a sulphidic phase. The enrichment factor of V is comparable to that of dysaerobic settings in Toarcian sediments and at the lower end of upwelling centres (Brumsack, 2006;Fantasia, Föllmi, Adatte, Spangenberg, & Montero-Serrano, 2018). They are substantially lower than those associated with the Cenomanian-Turonian oceanic anoxic episode (C-T OAE; Brumsack, 2006).…”

“…Uranium shows a strong correlation with P, indicating that a large fraction resides in authigenic phosphate (Table 1). Its enrichment factor is comparable to those in Toarcian dysoxic sediments, at the low end of those in recent upwelling centres, and clearly below those associated with sapropel and C-T OAE sediments (Brumsack, 2006;Fantasia et al, 2018). As such it would indicate dysoxic to anoxic conditions (Algeo & Maynard, 2004;Tribovillard et al, 2006).…”

Expression of the Toarcian Oceanic Anoxic Event: New insights from a Swiss transect

“…The T‐OAE coincides with global warming, sea‐level rise (Haq et al ., ; Hardenbol et al ., ) and an acceleration of the hydrological cycle, which increased detrital input (Cohen et al ., ; Dera et al ., ; Brazier et al ., ; Percival et al ., ; Them et al ., ; Fantasia et al ., ). At Riniken, Gipf (Jura) and Creux de l'Ours (Sub‐Briançonnais Basin), enhanced OM preservation (high TOC contents and HI values) during the T‐OAE is likely to be due to higher primary productivity in the surface water and/or the presence of oxygen‐poor conditions in the bottom waters.…”

“…The negative CIE is commonly attributed to the massive release of isotopically light carbon to the exogenic reservoirs, related to any one or a combination of the following processes: (i) CO 2 degassing during the formation of the Karoo–Ferrar large igneous province (LIP) (Duncan et al ., ; Pálfy & Smith, ); (ii) thermogenic methane (CH 4 ) release related to sill emplacement in the Karoo–Ferrar province (McElwain et al ., ; Svensen et al ., ); (iii) destabilization of CH 4 hydrates from marine sediments (Hesselbo et al ., ), modulated by orbital forcing (Kemp et al ., , ); and (iv) release of CH 4 from terrestrial environments (Pienkowski et al ., ; Them et al ., ). Furthermore, the onset of the Karoo–Ferrar LIP is likely to have triggered the change towards greenhouse conditions (Courtillot, ; Pálfy & Smith, ), which in turn intensified tropical cyclones and invigorated the hydrological cycle and continental weathering (Cohen et al ., ; Dera & Donnadieu, ; Suan et al ., ; Krencker et al ., ; Percival et al ., ; Them et al ., ; Fantasia et al ., ). This ultimately enhanced delivery of nutrients to the adjacent basins, boosting primary productivity and the development of anoxic conditions in the more restricted basins (Jenkyns, ; Them et al ., ).…”