ABSTRACT The evolution of the earliest complex state-level societies and cities from small sedentary communities took place in southern Mesopotamia between 8000 and 5000 cal yrs BP during the ‘Ubaid and Uruk periods. Attempts to explain this transition often discount the role of environmental change and tend to evaluate available archaeological evidence for urban-based state development either within a static environmental context or assuming conditions similar to those of the present. This practice is no longer tenable given newly available paleoenvironmental records for the region. Post-glacial sea-level rise resulted in the inundation and creation of the Arabo-Persian Gulf, and, as the marine transgression slowed in the Middle Holocene, rich coastal and aquatic habitats formed in southern Mesopotamia. These habitats favored the establishment and growth of ‘Ubaid Period communities and the efficient transport of goods, ideas, and people throughout the region. High water tables also promoted early experimentation with irrigation agriculture and the expansion of these systems as populations grew and the humid conditions of the Early Holocene gave way to increasing aridity. We argue that the critical confluence of eustatic and climatic changes unique to this circumscribed region favored the emergence of highly centralized, urban-based states.
During the Pliocene between ~5 and 3 Ma, polar ice sheets were restricted to Antarctica, and climate was at times significantly warmer than now. Debate on whether the Antarctic ice sheets and climate system withstood this warmth with relatively little change (stability hypothesis) or whether much of the ice sheet disappeared (deglaciation hypothesis) is ongoing. Paleoclimatic data from high-latitude deep-sea sediments strongly support the stability hypothesis. Oxygen isotopic data indicate that average sea-surface temperatures in the Southern Ocean could not have increased by more than ~3 °C during the warmest Pliocene intervals. A small rise in Southern Ocean temperatures may have caused limited melting of the ice sheets and associated marine transgression, but maximum sea level rise was likely less than 25 m above the present level. Recently discovered evidence from the Antarctic dry valleys indicate relative stability of the Antarctic climate-cryosphere system since middle Miocene time (~14 Ma).
A major cosmic-impact event has been proposed at the onset of the Younger Dryas (YD) cooling episode at ≈12,800 ± 150 years before present, forming the YD Boundary (YDB) layer, distributed over >50 million km2 on four continents. In 24 dated stratigraphic sections in 10 countries of the Northern Hemisphere, the YDB layer contains a clearly defined abundance peak in nanodiamonds (NDs), a major cosmic-impact proxy. Observed ND polytypes include cubic diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles, called n-diamond and i-carbon. The ND abundances in bulk YDB sediments ranged up to ≈500 ppb (mean: 200 ppb) and that in carbon spherules up to ≈3700 ppb (mean: ≈750 ppb); 138 of 205 sediment samples (67%) contained no detectable NDs. Isotopic evidence indicates that YDB NDs were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. The YDB layer is also marked by abundance peaks in other impact-related proxies, including cosmic-impact spherules, carbon spherules (some containing NDs), iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. This contribution reviews the debate about the presence, abundance, and origin of the concentration peak in YDB NDs. We describe an updated protocol for the extraction and concentration of NDs from sediment, carbon spherules, and ice, and we describe the basis for identification and classification of YDB ND polytypes, using nine analytical approaches. The large body of evidence now obtained about YDB NDs is strongly consistent with an origin by cosmic impact at ≈12,800 cal BP and is inconsistent with formation of YDB NDs by natural terrestrial processes, including wildfires, anthropogenesis, and/or influx of cosmic dust.
Abstract Multi-proxy analyses of a sequence spanning the Younger Dryas (YD) in the Glacial Lake Hind basin of Manitoba provides insight into regional paleohydrology and paleovegetation of meltwater rivers and lakes spanning >4000 yr; the sequence is controlled by 25 new accelerator mass spectrometry ages. This lake, dammed by the Laurentide Ice Sheet, overflowed into Lake Agassiz. The pre-YD interval records rapid sedimentation from meltwaters that headed in proglacial lakes in the Canadian Prairies that are known to have been catastrophically released when ice or sediment barriers were breached. Pollen in this phase is dominated by pre-Quaternary forms eroded from Paleocene bedrock. At the onset of the YD at ~12.8 cal ka, the sudden appearance of concentrations of nanodiamonds, high-temperature magnetic spherules, platinum, and iridium provide evidence of an extraterrestrial (ET) event that others have identified at more than 40 sites in North America. Major changes in oceans and climate, and the catastrophic outflow of nearby Lake Agassiz at the onset of the YD, may be related. Lower water levels and a reduction of Souris River inflow to Lake Hind followed, which are reflected by more clayey and organic-rich sediments and a decrease in pre-Quaternary palynomorphs. This may have resulted from the deepening of river valleys caused by the release of meltwater triggered by the ET event. Wetlands then began to develop, leading to peat deposition from 12.3 to 11 cal ka. This was followed by a fluvial episode depositing sand and then by increased Holocene aridity that resulted in accumulation of a thick sequence of dune sands. A dry woodland environment with a mix of conifers (especially Picea and Larix ) and deciduous trees (especially Populus and Quercus ) covered the uplands from ~13 to 10 cal ka.
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