The quaternary geography and biogeography of tidal saltmarshes

Climate change and sea-level change largely explain the changing distribution and structure of tidal saltmarshes over time, and these geographic attributes, in turn, are primarily responsible for the biogeography of tidal-saltmarsh organisms. This paper presents a general model of these relationships, and uses the San Francisco Bay-delta estuary (California) to demonstrate some of the model's implications and limitations. Throughout the Quaternary period, global cycles of glaciation and deglaciation have resulted in ca. 100-m variations in global mean sea level, which have been accompanied by large changes in the location of the intertidal coastal zone, and hence of potential sites for tidal marshes. Other climate-related variables (e.g., temperature and exposure to storms) have in turn substantially controlled both the location and size of marshes within the coastal zone and of specific physical environments (i.e., potential habitats) within marshes at any time. Since the most recent deglaciation resulted in a global rise in sea level of 100-130 m between about 21,000 and 7,000 yr BP, and a slower rise of about 10 m over the last 7,000 yr, modern tidal saltmarshes are relatively young geomorphic and ecological phenomena, and most continue to evolve in elevation and geomorphology. Therefore, the distribution of taxa between and within marshes reflects not only salinity and wetness at the time, the dominant controls on marsh zonation, but also antecedent conditions at present marsh sites and the extent and connectedness of habitat refugia during and since the glacial maximum. Unfortunately, direct stratigraphic evidence of paleomarsh extent and distribution is almost nonexistent for the Late Glacial-Early Holocene, and is incomplete for the late Holocene.