We present paleoclimate evidence for rapid ( < 100 years) shifts of f 2-4 jC in Chesapeake Bay (CB) temperature f2100, 1600, 950, 650, 400 and 150 years before present (years BP) reconstructed from magnesium/calcium (Mg/Ca) paleothermometry. These include large temperature excursions during the Little Ice Age (f1400 -1900 AD) and the Medieval Warm Period (f800 -1300 AD) possibly related to changes in the strength of North Atlantic thermohaline circulation (THC). Evidence is presented for a long period of sustained regional and North Atlantic-wide warmth with low-amplitude temperature variability between f450 and 1000 AD. In addition to centennial-scale temperature shifts, the existence of numerous temperature maxima between 2200 and 250 years BP (average f70 years) suggests that multi-decadal processes typical of the North Atlantic Oscillation (NAO) are an inherent feature of late Holocene climate. However, late 19th and 20th century temperature extremes in Chesapeake Bay associated with NAO climate variability exceeded those of the prior 2000 years, including the interval 450 -1000 AD, by 2 -3 jC, suggesting anomalous recent behavior of the climate system. D

We analyzed decadal and interannual climate variability in South Florida since 1880 using geochemical and faunal paleosalinity indicators from isotopically dated sediment cores at Russell Bank in Florida Bay (FB). Using the relative abundance of 2 ostracode species and the Mg/Ca ratios in Loxoconcha matagordensis shells to reconstruct paleosalinity, we found evidence for cyclic oscillations in the salinity of central FB. During this time salinity fluctuated from as low as ~18 parts per thousand (ppt) to as high as ~57 ppt. Time series analyses suggest, in addition to a 5.6 yr Mg/Ca based salinity periodicity, there are 3 other modes of variability in paleosalinity indicators: 6-7, 8-9, and 13-14 yr periods which occur in all paleo-proxies. To search for factors that might cause salinity to vary in FB, we compared the Russell Bank paleosalinity record to South Florida winter rainfall, the Southern Oscillation Index (SOI), winter North Atlantic Oscillation (NAO), and the winter Pacific North American (PNA) index, and a surrogate for the PNA in the winter season, the Central North Pacific (CNP) index. SOI and PNA/CNP appear to be associated with South Florida winter precipitation. Time series analyses of SOI and winter rainfall for the period 1910-1999 suggest ~5, 6-7, 8-9 and 13-14 yr cycles. The 6-7 yr and 13-14 yr cycles correspond to those observed in the faunal and geochemical time series from Russell Bank. The main periods of the CNP index are 5-6 and 13-15 yr, which are similar to those observed in FB paleosalinity. Cross-spectral analyses show that winter rainfall and salinity are coherent at 5.6 yr with a salinity lag of ~1.6 mo. These results suggest that regional rainfall variability influences FB salinity over interannual and decadal timescales and that much of this variability may have its origin in climate variability in the Pacific Ocean/atmosphere system. KEY WORDS: Paleoclimate · Florida Bay · Salinity · Shell geochemistry · ENSO · Pacific North American indexResale or republication not permitted without written consent of the publisher Clim Res 19: 233-245, 2002 els that simulate decadal patterns. For example, some model output and observational data suggest that causes of decadal variability occurring approximately every 20 yr reside in complex Pacific Oceanatmosphere interactions between tropical and midlatitude regions (Latif & Barnett 1994, Meehl et al. 1998, Barnett et al. 1999. Climate variability in the Pacific region associated with a global mid-to upper tropospheric disturbance was called the Pacific North American (PNA) pattern by Wallace & Gutzler (1981). More recently, the term Pacific Decadal Oscillation (PDO) has been applied to low-frequency oceanatmospheric variability originating in the Pacific region (Barnett et al. 1999). Similarly, surface oceanatmosphere interaction in the North Atlantic Ocean, called the North Atlantic Oscillation (NAO) (Rogers & van Loon 1979, Rogers 1984, is a dominant mode of winter climate variability in the North Atlantic region, and also ...

Studies of the seasonal ecology and shell chemistry of the ostracode Loxoconcha matagordensis and related species of Loxoconcha from regions off eastern North America reveal that shell size and trace elemental (Mg/Ca ratio) composition are useful in paleothermometry using fossil populations. Seasonal sampling of populations from Chesapeake Bay, augmented by samples from Florida Bay, indicate that shell size is inversely proportional to water temperature and that Mg/Ca ratios are positively correlated with the water temperature in which the adult carapace was secreted. Microprobe analyses of sectioned valves reveal intra-shell variability in Mg/Ca ratios but this does not strongly influence the utility of whole shell Mg/Ca analyses for paleoclimate application. Published by Elsevier B.V.