Terrestrial snails from archaeological sites as proxies for relative sea level on the Gulf Coast of Florida, USA

Abstract

Archaeological evidence for local environmental change is obscured by the tendency for humans to remove natural resources from places of procurement and deposit them elsewhere, sometimes at great distance. This is especially problematic for changes in relative sea level, which clearly affected the inhabitability of low-elevation coastal landforms but not necessarily the regional availability of resources of cultural or economic value. Needed are proxies for relative sea level from non-dietary taxa. One genus of terrestrial snails, Truncatella, offers good potential in this respect because of its specific niche at the interface between seawater and land. However, like food resources displaced by people, Truncatella shells are displaced by storms and redistributed landward of the coastline. Distinguishing between autochthonous and allochthonous deposits is essential to inferring relative sea level from the occurrence of this taxon alone. To this end, assemblages of Truncatella shell from stratified sites along the north Gulf Coast of Florida, USA are compared to associated archaeological snails of other taxa and to snail shells from the wrack of proximate foreshores to infer changes in relative sea level over the past four millennia. Variation in the morphology of shorelines and in the accumulation rates of archaeological midden mitigates any direct relationship between terrestrial snail frequencies and sea level, but the results of this study suggest that our approach can be applied to other non-dietary taxa occupying marginally terrestrial niches to refine estimates for sea level derived from the sedimentary records of geological cores.

Keywords:

• Sea-level change
• archaeomalacology
• coastal settlement

Acknowledgements

We are grateful to personnel of the Cedar Keys and Lower Suwannee National Wildlife Refuges of the US Fish and Wildlife Service (USFWS) for their enduring support of our efforts, notably Refuge Manager Andrew Gude, Deputy Manager Larry Woodward, and Fire Management Officer Vic Doig. USFWS Regional Historic Preservation Officer and Regional Archaeologist Richard S. Kanaski issued Archaeological Resources Protection Act (ARPA) permits (LSCKNWR 022113, LSCKNWR060614, and LSCKNWR060315) for fieldwork at North Key and Seahorse Key. University of Florida field school students in 2014 and 2015 unearthed the samples reported here, and additional student volunteers processed samples in the lab. Identifications and quantification of terrestrial snails were the handiwork of Caroline Steffy and Seth Shanefield. We are grateful to two anonymous reviewers and Editor Christina Giovas for suggestions to improve the clarity and relevance of this paper.

Supplemental Files

Supplement A: Stratigraphy and Chronology of Study Sites and Methods for Quantifying Terrestrial Snails.

Supplement B: Truncatella and Other Terrestrial Snails from Wrack.

Disclosure statement

No potential conflict of interest was reported by the authors.

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Notes

1 The wrack zone, or wrack line, is the accumulation of seagrass, shells, and other debris deposited along shorelines at high tide. Wrack zones are common to beach strands, but are also found along the edges of mangroves, salt marshes, and other coastal settings. A wrack zone forms when debris carried onshore by a rising tide is stranded in a line parallel to the shore as the tide recedes. The location of a wrack zone varies with short- and long-term changes in tidal range, and also by intermittent events like storms that carry debris farther landward than normal tides. Along sandy beaches, spring tides promote the development of erosional escarpments (i.e., wave-cut scarps) where wrack accumulates in large volume and can remain relatively stable until displaced by storms. It follows that the distribution and density of wrack is determined not only by tidal range and storm surge, but by the configuration of the foreshore (Barreiro et al. 2011).

2 Following the review of habitat for all terrestrial snails identified in this study, “marsh” species include Polygyra septemvolva (more specifically, marsh edge) and Melampus bidentatus. “Leaf litter” species refer to all taxa with wooded habitat or any niche involving leaf litter and/or fallen trees: Oligyra orbiculata, Hawaiia Sp., Lobosculum pustula, Gastrocopta contracta, Glyphyalinia umbilicata, Strobilops texasiana, Zonitoides arboreus. Archaeological sites on North Key are situated in wooded habitat of oaks, red cedar, and palm that extends to the shoreline and overhangs the wrack zone. The Gardiner’s Point site, in contrast, is more open, consisting of coastal scrub and palms, many fallen in recent years as the shoreline has continued to erode.

Additional information

Funding

Funding for this research came from the Hyatt and Cici Brown Endowment for Florida Archaeology, Department of Anthropology, University of Florida.