Zoogeomorphic Structures as Water-Level Indicators in Coastal Settings
Abstract
In addition to their substantial and largely underestimated role as landscape engineers and components of geomorphic cascades, a number of large organisms produce biogenic structures that may serve as reliable water-level indicators. This study reviews a suite of fluvial, lacustrine, and sea-level reference points that provide in situ benchmarks for reconstructing past hydrologic conditions. In areas where exposures or excavations are limited, near-surface geophysics (500-800 MHz georadar) provides noninvasive means of visualizing subsurface elements of biogenic structures and their relationship to hydrologic or paleoshoreline positions. 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). Typically, the upper extent of the phreatic zone (and, by extension, basin level) can be constrained between contemporary zoogenic structures formed above (mammal burrows; flamingo and guanay 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 (e.g., 50-60 cm for the Russian desman, Desmana moschata). 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. 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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP23C2298B
- Keywords:
-
- 0442 Estuarine and nearshore processes;
- BIOGEOSCIENCESDE: 1625 Geomorphology and weathering;
- GLOBAL CHANGEDE: 1641 Sea level change;
- GLOBAL CHANGEDE: 3020 Littoral processes;
- MARINE GEOLOGY AND GEOPHYSICS