The environment during the latest Pleistocene Settlement of the Americas

1 environment during latest pleistocene

1.1 emergence , submergence of beringia
1.2 glaciers
1.3 climate , biological environments
1.4 environmental changes during deglaciation

the environment during latest pleistocene

for introduction radiocarbon dating techniques used archaeologists , geologists, see radiocarbon dating.

emergence , submergence of beringia

figure1. submergence of beringian land bridge post-last glacial maximum (lgm) rise in eustatic sea level

during wisconsin glaciation, varying portions of earth s water stored glacier ice. water accumulated in glaciers, volume of water in oceans correspondingly decreased, resulting in lowering of eustatic sea level. variation of eustatic sea level time has been reconstructed using oxygen isotope analysis of deep sea cores, dating of marine terraces, , high resolution oxygen isotope sampling ocean basins , modern ice caps. drop of eustatic sea level 60 m 120 m lower present-day levels, commencing around 30,000 years bp, caused revealing of beringia durable , extensive geographic feature connecting siberia alaska. rise of sea level after last glacial maximum (lgm), beringian land bridge again submerged. estimates of final re-submergence of beringian land bridge based purely on present bathymetry of bering strait , eustatic sea level curve place event around 11,000 years bp (figure 1). ongoing research reconstructing beringian paleogeography during deglaciation change estimate , possible earlier submergence further constrain models of human migration north america.

glaciers

the onset of last glacial maximum after 30,000 years bp saw expansion of alpine glaciers , continental ice sheets blocked migration routes out of beringia. 21,000 years bp, , possibly thousands of years earlier, cordilleran , laurentide ice sheets coalesced east of rocky mountains, closing off potential migration route center of north america. alpine glaciers in coastal ranges , alaskan peninsula isolated interior of beringia pacific coast. coastal alpine glaciers , lobes of cordilleran ice coalesced piedmont glaciers covered large stretches of coastline far south vancouver island , formed ice lobe across straits of juan de fuca 15,000 c years bp (18,000 cal years bp). coastal alpine glaciers started retreat around 19,000 cal years bp while cordilleran ice continued advancing in puget lowlands 14,000 c years bp (16,800 cal years bp) during maximum extent of coastal ice, unglaciated refugia persisted on present-day islands, supported terrestrial , marine mammals. deglaciation occurred, refugia expanded until coast became ice-free 15,000 cal years bp. retreat of glaciers on alaskan peninsula provided access beringia pacific coast around 17,000 cal years bp. ice barrier between interior alaska , pacific coast broke starting around 13,500 c years (16,200 cal years) bp. ice-free corridor interior of north america opened between 13,000 , 12,000 cal years bp. glaciation in eastern siberia during lgm limited alpine , valley glaciers in mountain ranges , did not block access between siberia , beringia.

climate , biological environments

the paleoclimates , vegetation of eastern siberia , alaska during wisconsin glaciation have been deduced high resolution oxygen isotope data , pollen stratigraphy. prior last glacial maximum, climates in eastern siberia fluctuated between conditions approximating present day conditions , colder periods. pre-lgm warm cycles in arctic siberia saw flourishes of megafaunas. oxygen isotope record greenland ice cap suggests these cycles after 45k years bp lasted anywhere hundreds between 1 , 2 thousand years, greater duration of cold periods starting around 32k cal years bp. pollen record elikchan lake, north of sea of okhotsk, shows marked shift tree , shrub pollen herb pollen prior 26k c years bp, herb tundra replaced boreal forest , shrub steppe going lgm. similar record of tree/shrub pollen being replaced herb pollen lgm approached recovered near kolyma river in arctic siberia. abandonment of northern regions of siberia due rapid cooling or retreat of game species onset of lgm has been proposed explain lack of archaeosites in region dating lgm. pollen record alaskan side shows shifts between herb/shrub , shrub tundra prior lgm, suggesting less dramatic warming episodes allowed forest colonization on siberian side. diverse, though not plentiful, megafaunas present in environments. herb tundra dominated during lgm, due cold , dry conditions.

coastal environments during last glacial maximum complex. lowered sea level, , isostatic bulge equilibrated depression beneath cordilleran ice sheet, exposed continental shelf form coastal plain. while of coastal plain covered piedmont glaciers, unglaciated refugia supporting terrestrial mammals have been identified on haida gwaii, prince of wales island, , outer islands of alexander archipelago. now-submerged coastal plain has potential more refugia. pollen data indicate herb/shrub tundra vegetation in unglaciated areas, boreal forest towards southern end of range of cordilleran ice. coastal marine environment remained productive, indicated fossils of pinnipeds. highly productive kelp forests on rocky marine shallows may have been lure coastal migration. reconstruction of southern beringian coastline suggests potential highly productive coastal marine environment.

environmental changes during deglaciation

pollen data indicate warm period culminating between 14k , 11k c years bp (17k-13k cal years bp) followed cooling between 11k-10k c years bp (13k-11.5k cal years bp). coastal areas deglaciated rapidly coastal alpine glaciers, lobes of cordilleran ice, retreated. retreat accelerated sea levels rose , floated glacial termini. estimates of ice-free coast range between 16k , 15k cal years bp. littoral marine organisms colonized shorelines ocean water replaced glacial meltwater. replacement of herb/shrub tundra coniferous forests underway 12.4k c years bp (15k cal years bp) north of haida gwaii. eustatic sea level rise caused flooding, accelerated rate grew more rapid.

the inland cordilleran , laurentide ice sheets retreated more did coastal glaciers. opening of ice-free corridor did not occur until after 13k 12k cal years bp. environment of ice-free corridor dominated glacial outwash , meltwater, ice-dammed lakes , periodic flooding release of ice-dammed meltwater. biological productivity of deglaciated landscape gained slowly. earliest possible viability of ice-free corridor human migration route has been estimated @ 11.5k cal years bp.

birch forests advancing across former herb tundra in beringia 14.3ka c years bp (17k cal years bp) in response climatic amelioration, indicating increased productivity of landscape.