ZOOGEOMORPHIC INDICATORS OF (PALEO-)WATER-LEVEL POSITIONS

We present a review of fluvial, lacustrine, and sea-level reference points that provide in situ benchmarks for reconstructing past hydrologic boundaries. In areas where exposures or excavations are limited, near-surface geophysics (500–800 MHz georadar) provides non-invasive means of visualizing subsurface elements of biogenic structures and their relationship to water-level or paleoshoreline positions. Typically, the upper extent of the phreatic zone (and, by extension, the level of the water basin) can be constrained between contemporary zoogenic structures formed above (mammal burrows; flamingo and some cormorant ground nests) and below (catfish and some turtle burrows) the local water table. Partially submerged burrows of semiaquatic mammals can be used to constrain long-term stream and lake-level positions, their accuracy increasing with decreasing vertical dimension. With its lower part designed to be submerged, the gently dipping (height: 50–60 cm) burrow of the Russian desman (Desmana moschata) is an example of an excellent indicator. Lungfish and coeval tetrapod burrows can shed light on regional hydrologic fluctuations dating back to at least the Devonian. Along oceanic coasts, similar constrains can be provided by such supratidal structures as sea-turtle nests (e.g., egg-chamber base) and intertidal–subtidal feeding and dwelling traces with a known indicative meaning. Age dating of organic or clastic fractions associated with bioturbation structures (burrow fills, spoil piles) provides the chronological component of an index point (e.g., sea-level index point: SLIP). Some burrowing coastal mega-invertebrates that require hydration within their burrows are emerging as useful sea-level indicators in microtidal settings: e.g., accuracy ~5–10 cm for blue land crabs, Cardisoma/Discoplax spp. Radiocarbon dating of enclosed vegetation makes these burrows important SLIPs in late Cenozoic coastal sequences.