Thallium isotope cycling in a ferruginous Precambrian ocean analogue

Abstract

Precambrian oceans were overwhelmingly anoxic and rich in dissolved ferrous iron (Fe²⁺), conditions referred to as ‘ferruginous’. Yet, few paleoceanographic tools widely applied to the Precambrian sedimentary record are investigated in detail in the few available ferruginous settings today. We conducted a detailed thallium (Tl) isotope investigation of Deming Lake, a meromictic ferruginous lake in Itasca State Park (Minnesota, USA). Only slight changes were observed in dissolved Tl concentrations (Tl_diss) and isotopic compositions (ε²⁰⁵Tl_diss) between spring, summer, and winter months. This was despite the development of a winter ice cap, a dramatic shoaling of the oxycline, and large swings in dissolved and particulate Fe concentrations. Isotopic compositions of authigenic Tl (ε²⁰⁵Tl_A) leached from surface sediments across a shallow-to-deep gradient averaged –2.2 ± 0.9‱ (2SD) and were at all depths slightly lower than corresponding waters, which averaged 0.0 ± 1.3‱ (2SD). We estimate that the Tl isotope fractionation process responsible for driving this effect has an associated fractionation factor (α) of 0.9998 to 0.9999. Processes behind this fractionation could be preferential ²⁰³Tl removal by biomass and organic sulfur. Thallium removal from the uppermost water column during the spring and summer months had an associated α up to 1.0005, most likely due to the preferential sorption of ²⁰⁵Tl onto Mn oxide minerals. This process plays little to no role in setting sedimentary ε²⁰⁵Tl_A values, probably because Mn oxide minerals are unstable in sediments throughout the lake. We find no evidence of strongly coupled Tl and Fe cycling in Deming Lake, and by extension suspect that ferruginous conditions did not play an important direct role in ancient seawater Tl isotope mass-balance. What the lake does tell us, however, is that organic-rich and sulfur-poor sediments can slightly fractionate Tl isotopes. Ancient sediments formed under comparable conditions may be unreliable water column ε²⁰⁵Tl archives. And if these sediment types were widely distributed on the ancient seafloor, they could have exerted a minor effect on global seawater Tl isotope mass-balance. Our results provide novel insights into how Tl and its isotopes are cycled under ferruginous conditions, permitting more accurate application of the Tl isotope paleoredox proxy to the Precambrian sedimentary record.