Geochemical fingerprints of seawater in the Late Mesoproterozoic Midcontinent Rift, North America: Life at the marine-land divide

Abstract

The 1.1 Ga Midcontinent Rift (MCR) is a thick volcanic-sedimentary succession that forms a curvilinear belt through central North America and crops out along its northern apex around Lake Superior. Sedimentary units of the MCR have been long interpreted as fluvial-lacustrine and invited a number of studies on the early evolution of life in non-marine habitats. One of the key units is the siliciclastic Nonesuch Formation, thought to record deposition in a large lake. However, recent sedimentological observations indicate the presence of marine incursions. To further test this interpretation, we analysed trace element abundances in a broad suite of samples from multiple drill cores through the Nonesuch Formation. We aimed to differentiate geochemical influences of sediment provenance from post-depositional hydrothermal overprint and thereby identify authigenic enrichments in fluid-mobile elements that are indicators of primary environmental conditions. Our results reveal discrete enrichments in Mo and U in organic-and sulphide-rich horizons, which are most parsimoniously interpreted as marine signatures. This conclusion is supported by Sr/Ba ratios, which suggest mixing between freshwater and saltwater, and by rare cm-thick gypsum in the upper Copper Harbor Formation immediately below the Nonesuch rocks. The gypsum displays δ 34 S values of +25.9 ± 0.6‰, consistent with input of marine sulphate at least during parts of the basin's history. Collectively, our geochemical data support the sedimentological interpretation that this portion of the MCR archives a marine-influenced estuarine system. Although this conclusion rules out that microbial life documented from the MCR was living in exclusively freshwater habitats, the Nonesuch Fm and associated rocks still hold important clues about organisms that were capable of withstanding salinity gradients and bridging the gap between the marine and non-marine environments of the mid-Proterozoic.

Reconstructing Nitrogen Sources to Earth’s Earliest Biosphere at 3.7 Ga

“…A high Fe/Al ratio above ∼0.5 in sedimentary rocks, i.e., elevated relative to upper continental crust, is typically interpreted as evidence of anoxic conditions during the time of deposition that favored the accumulation of authigenic iron minerals, including sulfides, carbonates or oxides (Lyons and Severmann, 2006;Raiswell et al, 2019). However, in settings with significant contributions of iron-rich siliciclastic material or detrital iron sulfides or iron oxides, such as mafic rock-forming minerals and their weathering products, this empirically defined threshold is no longer applicable (Stüeken et al, 2017;Stüeken et al, 2020). It is therefore not possible to infer redox conditions during the time of deposition on the basis on Fe/Al ratios.…”

Far from boring: A new Grenvillian perspective on Mesoproterozoic tectonics

“…1). Furthermore, recent eld-based and geochemical studies of sedimentary rocks in the MCR documented wave-in uenced structures and marine sedimentation 26,27 .…”

“…In this new model, Laurentia was not involved in Grenvillian orogenesis until its latest stage. Laurentia was thus a passive margin throughout most of the Mesoproterozoic, with access to sea water such as documented in sedimentary rocks of the MCR 26,27 . The extremely fast motion of Laurentia between 1.1 and 1.08 Gyr-ago dragged by slab avalanche, would likely have put the entire continent under extension and generated all the major sedimentary basins and large igneous province such as the MCR that formed during this period 24,25 .…”

The Nonesuch Formation Lagerstätte: a rare window into freshwater life one billion years ago

“…These and other sedimentary fabrics may occur in a range of shallow water habitats and are not necessarily diagnostic of lacustrine settings. This, along with purported geochemical indicators of saline conditions, have led to proposals that the Nonesuch formed in shelfal (Jones et al 2020)or perhaps estuarine (Stüeken et al 2020) settings. However, new mineralogical studies focused on the redox chemistry of the Nonesuch environment continue to support a lacustrine interpretation (Slotznick et al 2018), whereas the geochemical evidence suggestive of marine conditions may actually be an artefact of the associated hydrothermal activity and copper mineralization.…”

“…Recent questioning of the lacustrine nature of the Nonesuch Formation (Jones et al 2020;Stüeken et al 2020) is a reminder of the difficulty in proving a freshwater origin for rocks that lack fossils of an unambiguous terrestrial provenance. However, biological clues can help to distinguish between marine and non-marine depositional settings.…”