Early Paleogene biosiliceous sedimentation in the Atlantic Ocean: Testing the inorganic origin hypothesis for Paleocene and Eocene chert and porcellanite

Abstract

The widespread occurrence of lower Eocene chert and porcellanite has been viewed as a major paleoceanographic issue since the advent of ocean drilling, and both biotic and abiotic forcings have been proposed to explain it. We present a reconstruction of indurated siliceous sediment (ISS) and preserved biosiliceous sediment (PBS) occurrences in the Atlantic Ocean through the Paleocene and Eocene (~66 through 34 Ma). ISS and PBS distributions reveal dissimilar temporal trends, with the peak of ISS occurrences coinciding with the Early Eocene Climatic Optimum, in line with previous studies. PBS occurrences show a generally increasing trend culminating between 44 and 43 Ma. The common co-occurrence of ISS and PBS, and their coherent geographic distribution lends strong support to the biogenic origin of the precursor to the widespread Paleogene ISS, and argues against an inorganic mode of early Cenozoic chert and porcellanite precipitation. Weight per cent biogenic opal records and trends in linear sedimentation rates indicate two plausible modes of silicification: 1) silicification due to prolonged exposure of 2 biogenic opal-rich sediments to corrosive bottom waters; and 2) silicification due to elevated pressures and temperatures caused by rapid burial of biogenic opal-rich deposits. The confinement of ISS and PBS to proximal sites along continental margins points to the reliance of siliceous sedimentation through the Paleocene and Eocene on terrestrial supply of dissolved silicon.Consistent with this, quantitative siliceous microfossil assemblage records from the Blake Nose in the NW Atlantic indicate that the nutrient-rich marginal rather than oligotrophic pelagic settings hosted the majority of siliceous plankton production through the early Paleogene.The inorganic SiO2 precipitation model is unlikely to have been the dominant mechanism responsible for ubiquitous occurrences of early Paleogene ISS. We favor the biogenic ISS precursor scenario and reconcile it with the low-productivity early Cenozoic oceans by showing that large volumes of biogenic silica were supplied to the western North Atlantic Ocean from the North American margin through the Paleocene and Eocene. Dissolution of this surplus silica was facilitated by an early southwestward flow of young, SiO2-depleted waters from the North Atlantic.

North Atlantic marine biogenic silica accumulation through the early-to-mid Paleogene: implications for ocean circulation and silicate weathering feedback

“…In contrast to the timing of NCW flow initiation indicated by isotopic proxy records, the onset of widespread 385 drift deposition in the North Atlantic is documented much earlier, i.e., near the termination of the EECO (~49 Ma; Hohbein et al, 2012;Boyle et al, 2017). This is also synchronous with ubiquitous deep-sea erosion coincident with the EMET (Aubry, 1995;Witkowski et al, 2020b), strongly suggesting that the onset of vigorous northern-sourced bottom current activity began at ~49-47 Ma (Vahlenkamp et al, 2018;Witkowski et al, 2020a). Following the EMET, the northward-flowing GSS and the invigorated deep WBC facilitated diapycnal mixing, which likely enhanced biological 390 pump efficiency along continental margins of the western North Atlantic.…”

“…Direct sediment measurements of bioSiO2 concentrations (with calculated fluxes) through the Paleogene are sparse (e.g., Diester-Haass, 1995;Salamy and Zachos, 1999;Diekmann et al, 2004;Lyle et al, 2005;Iwasaki et al, 2014), and mostly focus on restricted time windows of the late Eocene through early 105 Oligocene interval. A major reason for this is that bioSiO2 is highly vulnerable to water-column and seafloor dissolution, which results in early Paleogene siliceous phytoplankton occurrences often being confined to narrow stratigraphic intervals at many sites (see Barron et al, 2015;Witkowski et al, 2020b).…”

“…CC BY 4.0 License. framework is proposed for Site 1050 and 1051, and (2) Witkowski et al (2020b), in which Paleogene trends in chert 120 and porcellanite occurrences were compared to spatial and temporal patterns in Atlantic biosiliceous sediment occurrences. Here, we aim to determine the main controls on bioSiO2 fluxes in a key locus of biosiliceous accumulation in the Northwest Atlantic (Blake Nose) during the early-to-mid Paleogene -a period of Cenozoic climate change characterized by profound variations in global temperature.…”

“…include only a few narrow chert and porcellanite-bearing intervals (Witkowski et al, 2020b), and sparse clinoptilolite occurrences, which suggests minimal diagenesis of sedimentary bioSiO2. As such, the good overall preservation of siliceous microplankton in Paleogene sediments at BN, combined with the exceptionally long stratigraphic span of the record recovered during Leg 171B (Witkowski et al, 2020a, and Witkowski et al, 2020b). We use the age model of Witkowski et al (2020a), who showed that the Hole 1051A succession is interrupted by two major gaps that are broadly correlative to the hiatuses in Hole 1050A (see also Röhl et al, 2003).…”

“…Atlantic, especially along continental margins. The high diatom:radiolarian (D:R) ratios (Witkowski et al, 2020b, for further discussion see below) and common occurrence of well-preserved epiphytic diatoms such as Arachnoidiscus (see Witkowski et al, 2020a) suggest that much of BN bioSiO2 fluxes through the Paleocene may be attributed to neritic production. Varying proportions of continental runoff-derived versus upwelled nutrient input could also be invoked to explain the disparity in bioSiO2 fluxes between the more proximal Site 1051 and the more distal Site 1050 through the…”