Late Holocene variability in Florida Current surface density : patterns and possible causes

Abstract

Planktonic foraminiferal δ18O time series from three well-dated, high sedimentation rate cores near the Florida Keys (24.4°N, 83.3°W) exhibit repeated centennial to millennial-scale oscillations during the late Holocene. Isotopic shifts of 0.2–0.3‰ over the past 5200 years represent changes in sea-surface temperature (SST) of 1.0–1.5°C or salinity variability of 1–2 psu. The largest significant isotopic events are centered at approximately 200, 2000, 3200, and prior to 4000 calendar years BP. High Florida Current δ18O during the Little Ice Age (LIA) correlates with published records of high δ18O in the Sargasso Sea and low SST off the coast of west Africa. An interval of generally low δ18O in the Florida Straits from 1800 to 500 years BP is synchronous with the Medieval Warm Period off west Africa but leads low δ18O in the Sargasso Sea by several hundred years. Synchronous cooling across the subtropical gyre during the LIA is difficult to explain using interannual North Atlantic Oscillation patterns but may be consistent with the simulated effects of reduced solar irradiance. At frequencies between 1/1000 and 1/300 years during the Late Holocene, Florida Current δ18O is coherent with a published estimate of 14C production rate. Radiocarbon production seems to lead δ18O at these frequencies, but uncertainty in the phase calculation precludes a clear lead-lag relationship. At frequencies lower than 1/300 years, Florida Current δ18O is coherent and in phase with atmospheric Δ14C. The coherence of Δ14C and δ18O at periods >1000 years implies oceanic circulation may play a role in modulating atmospheric radiocarbon on millennial timescales.