Signatures of Quaternary sea level and climate in phreatic coastal caves PhD Candidate: Peter J. van Hengstum

Supervisor: Dr. David B. Scott Underwater (phreatic) caves are a ubiquitous landform on coastal karst terrain, but the marine geological processes operating in these systems are largely unknown. This dissertation redresses the problem by asking if Bermudian phreatic cave sediments archive sea-level and climate information? An important premise is that coastal cave environments are not identical. They can be categorized based on whether they are terrestrially-influenced (anchialine), completely flooded by saline groundwater (submarine), positioned at sea level (littoral) or in the vadose zone (vadose).

For the first time the boundary between modern anchialine and submarine cave environments has been distinguished in Green Bay Cave using a multi-proxy approach (benthic foraminifera, sedimentary organic matter content and carbon isotopic composition - δ13Corg, and grain-size analysis). Twelve push cores were extracted from Green Bay Cave and aged with twenty 14C dates, recovering the first underwater cave succession spanning the Holocene (13.1 ka to present). Green Bay Cave transitioned through all major cave environments during Holocene sea-level rise (vadose, littoral, anchialine, and submarine), providing a sedimentary model for global cave successions.

These relationships provide a novel means to solve Quaternary

sea-level and climate problems. For sea level, two examples indicate that the littoral cave can be used as a sea-level indicator, distinguished stratigraphically by microfossil or sedimentary proxies. First, the elevation and timing of when Green Bay Cave was a littoral environment indicates Bermuda experienced an abrupt 6.4 ±1.7 m sea-level rise at 7.6 ka, coinciding with final collapse of the Labrador sector of the Laurentide Ice Sheet. Second, microfossils (e.g., foraminifera, ostracods) preserved in elevated caves at +21 m above modern sea level and aged to marine isotope stage 11 (U-series, amino acid racemization) are consistent with modern Bermudian caves and co-stratigraphic sea level. For climate problems, annual temperature monitoring in Walsingham Cave indicates that cave water is thermally comparable to regional oceanographic conditions (e.g., Sargasso Sea temperature and Gulf Stream transport). Three sediment cores aged with sixteen 14C ages indicate that Bermuda’s coolest and stormiest conditions of the last 3.2 ka occurred during the little ice age (proxies: δ18Oc and bulk organic matter).