Thermally-anomalous Holocene molluscan assemblages from coastal Peru: evidence for paleographic, not climatic change

Abstract

Holocene marine mollusks, many in situ, are found in great numbers on anemergent coastal plain north of the Rio Santa, on the north-central coast of Peru. Samples of the mollusks were collected, identified, and grouped into assemblages of recurring species. The surficial sedimentology and geomorphology of the coastal plain were studied to determine the paleoecological significance of the molluscan shells, many of which belong to species now restricted to a biogeographically transitional zone north of Bayovar, Peru, 200 km north of the study of the area. Radiocarbon dates, mostly obtained from shells in natural deposits and middens, were used to track the Holocene development of the coastline. The warm-water mollusks lived within a shallow lagoon of about 29 km 2 between 4300–6500 yr B.P. Free tidal exchange with the Pacific Ocean was impeded by partially-drowned beach ridges within the lagoon and by a barrier-forming beach ridge that joined the mainland with an offshore island. Contemporaneus, in situ, cold-water faunas were discovered at the mouth of the lagoon and in washover deposits landward of the barrier beach ridge. The association of contemporaneous and adjacent assemblages of warm-water and cold-water mollusks shows that the thermally-anomalous molluscan assemblage (TAMA) at Santa is a consequence of local coastal paleographyy, not regional or global mid-Holocene climate change.