The Middle Pliocene ( approximately 3 million years ago) has been identified as the last time the Earth was significantly warmer than it was during the Last Interglacial and Holocene. A quantitative micropaleontological paleotemperature transect from equator to high latitudes in the North Atlantic indicates that Middle Pliocene warmth involved increased meridional oceanic heat transport.

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The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were > 0 øC and > 3 øC, respectively, and that ice-free conditions existed for most or all of the Arctic. By at least 3.5-3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such as Cythere, Hemicythere, and Neomonoceratina are known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and zoogeographic information on ostracode species from more than 800 modern "core top" samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 øC and 3 øC. This suggests ice-free summers, and probably, a perennially ice-free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the North Ariantic Ocean.Paper number 93PA00060. 0883-8305/93/93PA-00060510.00 INTRODUCTIONThe Arctic Ocean, in general, and Arctic sea ice, in particular, play an important but poorly understood role in oceanic circulation and global climate. Raymo et al. [1990] conducted an experiment using the GISS II atmospheric general circulation model (GCM) to examine the influence of reduced Arctic sea ice, a situation postulated for the Pliocene, on global climate. Among their conclusions, they suggested that late Pliocene cooling, well documented in the North Atlantic deep-sea record and many other paleoclimate proxy records, may have been linked in part to a shift in the Arctic Ocean from perennially ice-free to ice-covered conditions. However, they conclude that paleoclimate evidence from the Arctic is still too fragmentary to identify the forcing mechanisms that changed late Pliocene climate.Perhaps equally important as its influence on atmospheric parameters, Arctic Ocean sea ice contributes in a significant way to deepwater formation and the overall thermohaline circulation of the world's oceans [Aagaard, 1981; Aagaard et al., 1991; Rudels et al., 1991]. Freezing of sea ice and brine rejection in Arctic shelf seas leads to the formation of cold, dense water [Aagaard et al., 1985] and the maintenance of the modem Arctic Ocean halocline [Aagaard, 1981], which serves a key role as a heat sink and which buffers Arctic sea ice from underlying warm Arctic Intermediate Water (AIW) = (the Ariantic layer). A catastrophic breakdown of the halocline may have occurred during Termination I when rapid runoff lowered salinity so much that convection and deep ventilation stopped [Rooth, 1982; Broecker et al., 1985], Smaller-scale oscillations in fresh water from sea ice over historical times can similarly alter convection and deepwater formation [Aagaard and Carmack, 1989] because th...

In the present study, we compare modern and Quaternary ostracods from two lacustrine basins: Laguna Cari-Laufquen (41°S) and Lago Cardiel (49°S) in Patagonia. Taxonomic and quantitative analyses along with isotopic and chemical studies of the extant ostracod fauna indicate that distinct ostracod associations can be identified as a function of conductivity. Three ostracod associations can be distinguished: (1) springs, ponds and small creeks, characterized by low conductivity (e.g. 1015 ms cm -1 ); (2) lakes and permanent ponds, characterized by medium conductivity (e.g. 1625 ms cm -1 ) and (3) ephemeral lacustrine environments, generally characterized by higher conductivity (e.g. 16 480 ms cm -1 ) These modern ostracod associations were also identified in older sequences from sediments outcropping in the Laguna Cari-Laufquen current shoreline, as well as in sediment cores from Lago Cardiel. The predominance of Limnocythere rionegroensis Cusminsky & Whatley in the Cari-Laufquen sections suggests the development of a saline and turbid lake during the Late Pleistocene and Early Holocene, and thus higher precipitation at these latitudes. Changes in ostracod abundance and associations have been observed in Lago Cardiel during the last approximately 16 000 calibrated years BP. Conductivity is known to change as a function of the ratio of precipitation to evaporation and a decrease in conductivity from the Late Pleistocene to the Middle Holocene suggests substantial hydrological variations (i.e. increase of the precipitation/evaporation ratio suggests minor conductivity). These two examples show that ostracods provide an excellent proxy for interpreting palaeoclimatic and palaeoenvironmental changes in Patagonia.

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The acheulian site at Boxgrove contains one of the most extensive areas of in situ fauna and flintwork yet discovered in Britain. This material is found in a complex sequence of sediments which represent depositional conditions from a 42 m sea level rise to the onset of a full periglacial climate. Excavation of the archaeological horizon has been accompanied by a programme of multidisciplinary research examining site formation processes, palaeolandscape and palaeoecological development, using sedimentological and environmental reconstruction techniques. Dating of the site is tentative as no absolute dates are available at present. However, comparative analysis with other British sites would suggest a position for the Boxgrove sequence within the Middle Pleistocene. The archaeological horizon is interpreted as being deposited towards the latter part of an interglacial or an interstadial period.

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