Was Delayed Intercontinental Seas' Euxinia A Vital for Eukaryotes to Survive During the Statherian PaleoProterozoic?

Delayed euxinia in Paleoproterozoic intracontinental seas: Vital havens for the evolution of eukaryotes?

Authors:

Spinks et al

Abstract:

Increased flux of sulfate to the oceans in the aftermath of the Great Oxidation Event (GOE) ∼2.4 billion years ago (Ga) caused major changes in seawater chemistry, which eventually contributed to the cessation of iron formation deposition ∼1.8 Ga. It is generally accepted that this engendered heterogeneous stratified redox conditions, with anoxic and sulfidic (euxinic) conditions in shallow open-marine environments and anoxic ferruginous conditions in deeper environments. However, the redox evolution of intracontinental marine basins following the cessation of iron formation deposition remains poorly understood.

Here, we report contrasting paleoredox conditions in two shale units of the lower McArthur Basin, northern Australia, soon after the cessation of iron formation deposition ∼1.84 Ga. Our data shows that the ∼1.78 Ga McDermott Formation was deposited in a sulfur-limited, anoxic shallow-marine environment, whereas the younger ∼1.73 Ga Wollogorang Formation was deposited in a euxinic shallow-marine environment. This implies a delay in the development of euxinia in a shallow intracontinental basin following the onset of euxinia in the open marine realm. Since bioessential metals are sequestered by pyrite deposition under euxinic conditions, protracted low-sulfidic conditions in 1.78 Ga intracontinental shallow environments could have provided vital niches for nitrogen-fixing prokaryotes and eukaryotes. Thus the ability for localized Paleoproterozoic intracontinental basins to remain non-euxinic after the onset of euxinia in shallow open-marine shelves highlights the importance of intracontinental environments to the evolution and diversification of microbial life, perhaps throughout the wider Proterozoic.

Evidences for a Paleocene Paleogene Marine Incursion in Southern Amazonia

Evidences for a Paleocene marine incursion in southern Amazonia (Madre de Dios Sub-Andean Zone, Peru)

Authors:

Louterbach et al

Abstract:

This article presents new biostratigraphic dating, facies analysis, organic geochemical data and Nd–Sr isotopic provenance from five outcrops of southern Amazonia (MD-85, MD-177 MD-184, MD-255 and MD-256) to document for the first time the presence of a shallow marine ingression in the Paleocene of southern Amazonia basin. The co-occurrence of a selachian assemblage encompassing Potobatis sp., Ouledia sp., and Pristidae indet. with the ostracod Protobuntonia sp. and the charophytes Peckichara cf. varians meridionalis, Platychara perlata, and Feistiella cf. gildemeisteri suggests a Paleocene age for the studied deposits (most likely Thanetian but potentially Danian). Fifteen facies have been recognized and have been grouped into three facies assemblages. Facies association A corresponds to the sedimentary filling of a tide-influenced meandering channel formed in the fluvial–tidal transition zone. Facies association B is related to more distal tidal-flats, little channelized tidal inlets and saltmarsh deposits. Facies association C corresponds to a stressed shallow marine environment such as a bay or a lagoon. The δ13CTOC value (− 23.4‰) of MD-184 is enriched in 13C compared to the other samples suggesting the presence of substantial amounts of marine organic matter in MD-184. The δ13CTOC values of samples from other outcrops (− 27.3 to − 29.8‰) indicate a mixed organic matter origin, from terrestrial to brackish environments. The analyzed sediments have similar Nd–Sr isotopic compositions as those of the Cenozoic sediments of the Altiplano (εNd(0) values from − 6.2 to − 10.7 and 87Sr/86Sr compositions from 0.712024 to 0.719026) indicating a similar volcanic source. This multidisciplinary dataset documents the presence of a tide-dominated estuary sourced by the proto-Western Cordillera debouching into a shallow marine bay during Paleocene times. This transgression might be explained by subsidence created in response to the proto-Western Cordillera loading. Similar to Miocene marine incursions affecting the Pebas megawetland, Paleogene marine incursions in the Amazonian foreland basin associated with Andean uplift may have played a role in the Neotropical biodiversity dynamics in favoring biogeographical isolation and promoting allopatric speciation for terrestrial organisms.

Paleocene–Eocene Thermal Maximum's Effect on the Epicontinental West Siberian Sea

Paleocene–Eocene warming and biotic response in the epicontinental West Siberian Sea

Authors:

Frieling et al

Abstract:

We present a Paleocene–Eocene (ca. 60–52 Ma) sea-surface temperature record from sediments deposited in the epicontinental West Siberian Sea. TEX86 paleothermometry indicates long-term late Paleocene (∼17 °C ca. 59 Ma) to early Eocene (26 °C at 52 Ma) sea-surface warming, consistent with trends previously observed for the Southern Ocean and deep oceans. Photic zone and seafloor anoxia developed as temperatures rose by 7 °C to ∼27 °C during the Paleocene–Eocene Thermal Maximum (PETM). Based on paired palynological and TEX86 data, we suggest that the minimum temperature for the proliferation of Paleocene and early Eocene members of the dinoflagellate family Wetzelielloideae, which includes the PETM marker taxon Apectodinium, was ∼20 °C.