Si isotope variability in Proterozoic cherts

Abstract

We report Si-isotopic compositions of 75 sedimentologically and petrographically characterized chert samples with ages ranging from $2600 to 750 Ma using multi-collector inductively coupled plasma mass spectrometry. d 30 Si values of the cherts analyzed in this study show a $7& range, from À4.29 to +2.85. This variability can be explained in part by (1) simple mixing of silica derived from continental (higher d 30 Si) and hydrothermal (lower d 30 Si) sources, (2) multiple mechanisms of silica precipitation and (3) Rayleigh-type fractionations within pore waters of individual basins.We observe $3& variation in peritidal cherts from a single Neoproterozoic sedimentary basin (Spitsbergen). This variation can be explained by Rayleigh-type fractionation during precipitation from silica-saturated porewaters. In some samples, postdissolution and reprecipitation of silica could have added to this effect. Our data also indicate that peritidal cherts are enriched in the heavier isotopes of Si whereas basinal cherts associated with banded iron formations (BIF) show lower d 30 Si. This difference could partly be due to Si being derived from hydrothermal sources in BIFs. We postulate that the difference in d 30 Si between non-BIF and BIF cherts is consistent with the contrasting genesis of these deposits. Low d 30 Si in BIF is consistent with laboratory experiments showing that silica adsorbed onto Fe-hydroxide particles preferentially incorporates lighter Si isotopes.Despite large intrabasinal variation and environmental differences, the data show a clear pattern of secular variation. Low d 30 Si in Archean cherts is consistent with a dominantly hydrothermal source of silica to the oceans at that time. The monotonically increasing d 30 Si from 3.8 to 1.5 Ga appears to reflect a general increase in continental versus hydrothermal sources of Si in seawater, as well as the preferential removal of lighter Si isotopes during silica precipitation in iron-associated cherts from silica-saturated seawater. The highest d 30 Si values are observed in 1.5 Ga peritidal cherts; in part, these enriched values could reflect increasing sequestration of light silica during soil-forming processes, thus, delivering relatively heavy dissolved silica to the oceans from continental sources. The causes behind the reversal in trend towards lower d 30 Si in cherts younger than 1.5 Ga old are less clear. Cherts deposited 1800-1900 Ma are especially low d 30 Si, a possible indication of transiently strong hydrothermal input at this time.

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…Though the data are noisy, Precambrian cherts record δ 30 Si of ∼0‰ in the Archean, rising to ∼2‰ in the Mesoproterozoic-with some isolated individual values of >4‰-before declining to Archean values in the late Neoproterozoic/early Phanerozoic. This long-term pattern has been noted before (e.g., Robert and Chaussidon, 2006;Chakrabarti et al, 2012;Chakrabarti, 2015) but remains enigmatic. Given that the BIF and chert δ 30 Si record is one of the few windows to Precambrian Si cycling, understanding its drivers should be a priority.…”

“…Given that the BIF and chert δ 30 Si record is one of the few windows to Precambrian Si cycling, understanding its drivers should be a priority. Robert and Chaussidon (2006) interpret the data as variable fractions of ocean Si removed by hydrothermal silificication, while Chakrabarti et al (2012) favor a combination of different processes acting in concert.…”

“…Frontiers in Marine Science | www.frontiersin.org Robert and Chaussidon (2006), van den Boorn et al (2007van den Boorn et al ( , 2010, Steinhoefel et al (2009Steinhoefel et al ( , 2010, Abraham et al (2011), Heck et al (2011), Chakrabarti et al (2012, Delvigne (2012), Marin-Carbonne et al (2014), Hu et al (2013), Ramseyer et al (2013), Geilert et al (2014), Li et al (2014), Brengman (2015, Stefurak et al (2015), Delvigne et al (2012), Wen et al (2016), and Ding et al (2017).…”

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…. ) associated to silica in Banded Iron Formations (e.g., André et al, 2006;Heck et al, 2011;Delvigne et al, 2012), cherts (e.g., Robert and Chaussidon, 2006;Chakrabarti et al, 2012) or paleosols (e.g., Delvigne et al, 2016). These studies all use the knowledge gained from modern, experimental and/or less ancient palaeorecords to interpret geochemical records, in order to reconstruct, for instance, the ocean temperature and oxygen levels, input of hydrothermal, and intensity of weathering.…”

Paleobiological Perspectives on Early Eukaryotic Evolution

“…Early diagenetic cherts, a rich source of microfossils in Proterozoic strata, are largely barren, perhaps reflecting the strong influence of hydrothermal fluid flow and iron deposition on the silica cycle of Archean oceans (Fischer and Knoll 2009;Chakrabarti et al 2012). Shales, in turn, contain abundant organic carbon, but few structurally preserved or morphologically distinctive microfossils.…”