Did you say ‘fast’? Mysterious early diagenesis in sub-recent lacustrine sediments of Lake Van

Soft, unlithified sediments recovered from modern lakes rarely offer clear evidence of diagenetic alterations. Recent work has documented products of early diagenesis in the deep lacustrine setting of Lake Van. Lake Van, cored in 2010 in the frame of the ICDP PALEOVAN project, is a terminal, alkaline lake in Eastern Anatolia, Turkey (McCormack & Kwiecien, 2021). The lake carbonate inventory consists of (1) primary phases: inorganic calcite and aragonite precipitating in surface water, and low-Mg calcite ostracod valves formed at the sediment-water interface; and (2) secondary phases: early diagenetic dolomite forming in the sediment pores and aragonite encrustation of ostracod valves and organic remains.  Here we focus on aragonite encrustations.

Encrusted grains appear episodically in Lake Van sediments younger than 270 ka, and their occurrence is restricted to two lithologies; homogenous and banded muds, representing lake low-stands, reduced primary productivity/preservation and a well-ventilated water column. Although lake level changes occurred in the past, the water depth of the coring site – today at 350 m – unlikely fell below 200 m.

SEM and thin section analyses of the as yet enigmatic encrustations show two generations of aragonite crystals; larger (10 – 20 μm), columnar to blocky ones (inside the closed valves) and a magnitude smaller (1 – 2 μm), columnar ones (outside the valves) intercalated with clay minerals and probably organic matter. The isotopic composition of encrusted valves contrasts with that of inorganic carbonates precipitating in the water column; higher δ¹⁸O values support a formation in cold bottom water, higher δ¹³C values are likely related to microbial activity, however, the nature of this relation is yet unclear. Encrusted valves are often articulated but display different stages of opening. As ostracod valves usually disarticulate within hours to days after the animal’s demise, semi-open valves suggest that the early diagenetic process was – in geological terms – extremely rapid.

Our finding calls for care and attention analyzing even sub-recent biogenic carbonates. The episodic and facies-bound occurrence suggests that encrustation is ultimately controlled by environmental factors, yet so far, we were unable to pinpoint these factors or a mechanism responsible for this process. If you are intrigued just like us, do get in touch!  

 

References

McCormack & Kwiecien, 2021. Coeval primary and diagenetic carbonates in lacustrine sediments challenge palaeoclimate interpretations. Scientific Reports