Palaeoenvironmental interpretations of the A4 stromatolites from the Koobi Fora Formation in Lake Turkana, Kenya
Creators: name: LaCarrubba, Jonathon M.
Publisher: name: No Publisher Supplied
Year: 2018

description: The stromatolites of the A4 horizon in the Koobi Fora Formation in Lake Turkana, Kenya have predominantly been utilized as a marker bed horizon by researchers since the early 1980s to resolve questions about the sedimentary environments before and after layer precipitation around 1.616 MYA, but little is known about the actual environments in which the layer precipitated. This study interpreted the stable isotopes of two A4 stromatolites through the lens of Pleistocene stromatolite morphologies, extant stromatolitic environments, and recent developments into Lake Turkana delta^18Owater values to conduct a palaeoenvironmental reconstruction of the A4 stromatolite horizon.     The stromatolites studied here were collected from areas previously identified as the likely locations of the palaeo-Omo River delta and an outlet to the Indian Ocean called the Turkana River (Brown and Feibel, 1991). A negative correlation between the water temperatures calculated from the delta^18Ocalcite of these stromatolites and the water column depth interpreted from the stromatolite morphologies confirms the previous conclusions about the locations of this basin inlet and outfall respectively. The A4 stromatolite horizon developed from a floodplain environment that was connected to the larger lake body with a shallow water column and low energy to a through-flowing water environment with a deeper water column and slightly more energy. The stromatolite producing organisms that precipitated the specimen studied here ultimately die during a period of local sedimentary infilling in an environment of shallow, low energy, higher-temperature water.  While there is debate as to whether the Lake Turkana Basin has contained a lake consistently throughout its history, this research demonstrates that a Pleistocene basin lake could not be the sole environment of precipitation and preservation of the A4 stromatolite horizon. This analysis of the A4 stromatolite morphology and stable isotopes underscores an environment of variable water column depth and water energy.     Changes in local evaporation, alkalinity shifting, and therefore delta^18Owater, complicate temperature calculations from the delta^18Ocalcite of stromatolites. Furthermore, stromatolite producing communities dissolve and recycle extracellular polymeric substance (EPS) in response to pH shifts, seasonal sediment cycles, temperature decreases, and photic zone changes, underscoring the complexity of temperature calculations using stromatolite delta^18Ocalcite. These kinetic or environmental effects are not well constrained; however, by deriving an ideal delta^18Owater for stromatolite producing temperatures, this study identifies that with a shift of about 1 to 2permil$ it is possible to keep stromatolite precipitation within realistic ranges for stromatolitic growth. This study concludes that in the Koobi Fora collection areas of 107 and 123, the A4 stromatolites represent a period of variable environmental conditions that began as a floodplain and developed into a predominantly through-flowing water environment until local sedimentation infilled the horizon. This research also underscores the fact that stable isotopes derived from stromatolites are less than ideal for temperature calculations due to the biological and kinetic growth processes of stromatolite producing microbial communities.