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Salinity Gradients and Catastrophic Freshwater Stress on Incipient Estuarine Ichnofossil Assemblages—Trinity Bay Head Delta: A Key to Defining Modern and Ancient Brackish Water Ichnofacies


In late September, 2012, after 1 year of drought and little to no freshwater runoff in the Trinity River basin, saltwater incursion 10.7 km upstream from the mouth of the modern Trinity River made the Wallisville distributary channel 9 ppt, and delta front of the Trinity River bay head delta 12–15 ppt. Small- to medium-sized Ocypode (ghost crabs) were present along the shore constructing incipient burrows assignable to Skolithos and Psilonichnus. Taegelus (razor clams), juvenile Lepidophthalmus (mud shrimp), and polychaetes colonized the intertidal flats and constructed burrows assignable to Siphonichnus, Ophiomorpha, Thalassinoides, Skolithos, Polykladichnus, Cylindrichnus, and Palaeophycus, respectively. This community persisted until 2016, when massive precipitation events flushed freshwater into the bays and estuaries. Salinity levels in the Trinity River and the upper part of the bay dropped abruptly to 0 ppt following a major storm and runoff in March, 2016, and persisted through much of the spring and summer. The freshwater killed Taegelus, with no live specimens collected during exploratory digging, although many dead, articulated specimens were observed in life position at the bottom of their vertical siphon burrows. Lepidophthalmus was not present, live or dead, indicating that they too, were all dead, but possibly not preserved in their burrows, and a single burrow of Ocypode was located. Polychaetes were still present, in greater numbers than 2012, and consisted of the predatory Glycinde and the bloodworm Glycera. These polychaetes are overprinting existing bivalve and arthropod burrows and provide a possible means for recognizing episodes of freshwater stress on marine communities in outcrop and subsurface analogs. Additionally, the stunting and diminutive nature characteristic of brackish water ichnofossil assemblages may be more a function of episodic mass death of juvenile benthic organisms, than actual dwarfing of adults in response to environmental stress.