Marinecore research

The study of marine cores and their contained fossil assemblages and the stable-isotope analyses of foraminifers revolutionized our understanding of Quaternary climate history with the demonstration that cyclical variations in the Earth's orbit around the Sun are the pacemakers of glacial-interglacial cycles (Hays et al. 1976; Imbrie and Imbrie 1979). In contrast, the contributions of marine research to Holocene climate history have been relatively small, compared with the contributions from terrestrial, limnologic, and peat records and from ice-core records. Jansen et al. (2004) provide an overview of Holocene climate variability from a marine perspective.

Perhaps the most influential Holocene marine studies have concerned the occurrence of ice-rafted debris (hematite-stained grains, Icelandic volcanic glass shards, and detrital carbonate) in cores from the North Atlantic (Bond et al. 1997, 2001), and the suggestion that this debris occurs with periodicities of 1500 years, related to unknown internal oscillations of the climate system (Bond et al. 1997) or to variations in solar output throughout the Holocene (Bond et al. 2001). Similar 1500-year periodicities have been reported from, for example, sediment grain-size data from a North Atlantic core, considered to be a proxy for the speed of deep-water flow (Bianchi and McCave 1999). In addition to a 1500-1600-year periodicity in sediment color data from a North Atlantic core (a possible proxy for North Atlantic Deep Water circulation), Chapman and Shackleton (2000) report 550-and 1000-year periodicities. The existence of millennial-scale periodicities during the Holocene has been questioned by, for example, Schulz and Paul (2000) and Risebrobakken et al. (2003). The detection of periodicities from geologic core data is fraught with difficulties - sampling resolution, irregular sample distribution in time, time control, and the reliability and inherent errors in the resulting age-depth models. Time control is particularly difficult in marine studies due to spatial and possible temporal variations in the marine reservoir effect and the effects of these variations on radiocarbon datings in the Holocene.

Multi-proxy high-resolution analyses of rapidly sedimented marine cores from the North Atlantic (e.g. Birks and KoƧ 2002; Andersson et al. 2003; Risebrobakken et al. 2003; Andersen et al. 2004; Moros et al. 2004) involving reconstructions of sea-surface temperatures from fossil assemblages and alkenone unsaturation ratios, stable-isotope analyses, and ice-rafted debris suggest four major climatic phases in the Holocene. There is an initial early Holocene thermal maximum that lasted to about 6700 years ago, followed by a distinct cooler phase associated with increased ice rafting between 6500 and 3700 years ago. There was then a transition to generally warmer but relatively unstable conditions between 3700 and 2000 years ago. Then there was a striking decline in sea-surface temperature between 2000 and 500 years ago. Although the dominant forcing factor for the early Holocene climate history was Northern Hemisphere summer insolation via strong seasonality at high northern latitudes, the trigger for the onset of relatively unstable climatic conditions beginning about 3700 years ago is less clear. Moros et al. (2004) suggest that this change may have been triggered by a late Holocene winter insolation increase at high northern latitudes and/or interhemispheric changes in orbital forcing. The late Holocene Neoglaciation trend beginning about 2000 years ago, which is a feature of many terrestrial records in northern Europe, may have been driven not only by a gradual decrease in orbitally forced summer temperatures but also by an increase in snow precipitation at high northern latitudes during generally milder winters. Such high-resolution multi-proxy marine studies highlight the role of a range of forcing functions and close ocean-terrestrial links. Jansen (this volume) reviews North Atlantic climate and ocean variability at different temporal scales.

In terms of paradigms in Holocene climate research, marine studies clearly show that climate change is frequent in the Holocene but whether it follows regular periodicities is presently unresolved.

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