185-6 Burial Speed Relative to Littoral Gradient Position Controls Photopigment Diagenesis in Back-Barrier Microbial Mats

Session: New Advances and Voices in Geobiology (Posters)

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Microbial mats endure oxidative and nutrient-related stresses borne from intensive diel redox gradients combined with periodic stranding from seawater. This study quantifies unique mat photopigment and photopigment diagenetic products concomitant to dynamic physiochemical conditions influenced by elevation gradients relative to sea level.  Two 65 cm push cores were collected at three locations in Mustang Island, Texas, each corresponding to an elevation at which the microbial mats were saturated during low tide, mid tide, and high tide respectively.  Photopigments were quantified using tandem reversed phase high pressure liquid chromatography – atmospheric pressure chemical ionization ion trap mass spectrometry. Photopigments were identified using a combination of UV/VIS spectra and mass to charge ratios. Sediment packages and associated relict mats were dated using optically stimulated luminescence, with equivalent dose calculated on ultra-small pure quartz fractions. The age model revealed a dynamic sedimentation history, with the bulk of the upper 30 cm deposited in the last 30 years in the low- and mid-tide positions, with maximum ages approximating 800 years. The high tide positions were especially defined by rapid sedimentation, with the oldest sediments reaching 160 years. The cyanobacterial sheath pigment scytonemin was the most ubiquitous and long-lasting pigment, found in both modern mats and throughout the oldest depths of all but the oldest high-tide cores. Reduced scytonemin was present in lower concentrations within low- and mid-tide cores, consistent with more frequent saturation and consequently less exposure to oxic conditions. Hydroxylated photopigment derivatives of both oxygenic and anoxygenic photosynthetic organisms were detected alongside their precursors, with the anoxygenic pigments less prevalent among the high tide positions. The hydroxylated products, shown to be produced upon reactions with reactive oxygen species, were most abundant within the mid-tide positions. This may indicate prolonged oxidative stress within microbial mats that are highly productive during moderate saturation periods followed by desiccation.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10659

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