The Dziani Dzaha Lake: A long‐awaited modern analogue for superheavy pyrites

Abstract

Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive δ34Spyrite values together with higher δ34Spyrite than coeval δ34SCAS) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate‐poor environments. We report here the first observation of strongly positive δ34S values for both dissolved sulfate and sulfide (average δ34Sdiss.sulfate value of 34.6‰ and δ34Sdiss.sulfide values of 36.7‰) compared to the present‐day seawater δ34Sdiss.sulfate (~21‰), with a negative apparent fractionation between sulfate and sulfide (∆34Sdiss.sulfate‐diss.sulfide ~ −2.1 ± 1.4‰), in the sulfate‐poor (<3 mm) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative ∆34Ssed.sulfate‐sed.sulfide (−4.98 ± 4.5‰), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two‐stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate‐rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in 34S the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic–anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate‐poor and highly productive environments analogous to the Dziani Dzaha.

Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake

“…Vertical profiles of [DIC], δ 13 C DIC , δ 13 C POC and δ 15 N PON in the Dziani Dzaha Lake water column. The grey profiles correspond to previous data obtained before 2018 from Cadeau [2017] and Cadeau et al [2020Cadeau et al [ , 2021Cadeau et al [ , 2022.…”

“…Vertical profiles of physical and chemical parameters recorded during each survey including salinity (psu), temperature (°C), pH and dissolved oxygen (%). The grey profiles correspond to previous data obtained before 2018 from Cadeau et al [2022].…”

“…The water column was periodically stratified with a sharp halocline at ca. 2 m depth about half of the year, driven by the salinity decrease in surface waters as a result (A) Mayotte Islands in Western Indian Ocean, (B) Dziani Dzaha Lake in Petite Terre Island on Mayotte, (C) Picture of the lake taken in October 2014, and (D) Bathymetric map of the lake, with blue circle representing the water column sampling location and the red circles representing the identified bubbling areas at the water-air interface (modified from Leboulanger et al [2017] and Cadeau et al [2022]). of increasing precipitation during the rainy season.…”

“…For instance, based on water volume calculations [e.g. Cadeau et al, 2022], an increase of the water level of about 20 cm (corresponding to the subsidence of Mayotte Island) would represent a water input of about 7.2% of the total water volume (V input in ( 1)). From this, the calculated salinities considering seawater or freshwater inputs are close to 61.4 psu or 58.9 psu, respectively.…”

“…Previous studies have shown that the strongly positive δ 13 C DIC in the Dziani Dzaha Lake was the consequence of organic matter degradation by methanogenesis associated with methane degassing into the atmosphere, which results from the complete sulfate consumption in an anoxic water column [Cadeau et al, 2020[Cadeau et al, , 2022. In contrast to other lacustrine systems in which methanogenesis has been suggested to explain 13 C enrichment observed in the sediment record or in the water column [Talbot and Kelts, 1986, Valero-Garcés et al, 1999, Gu et al, 2004, Zhu et al, 2013, Birgel et al, 2015, the Dziani Dzaha Lake exhibits similar carbon isotope signatures in the lake waters and in the sediment, as well as a long-term steady state [Cadeau et al, 2020].…”