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14c carbon 14 for dating prehistoric findings

They discovered a strong association between expansions of northern hemisphere polar atmospheric circulation systems and the 2500-year cycle previously described by his former teacher (O’Brien et al., 1995; figure 46 F & G; figure 52 a & b). Windy periods, indicated by the transport and deposition of coarse sediments, are coeval with cool, stormy periods recorded in GISP2 ice and North Atlantic sediment cores. The Holocene NAO patterns have been reconstructed from a marine sediment core whose alkenone content has been shown to depend on trade winds intensity-dependent upwelling near the coast of NW Africa (Kim et al., 2007; figure 52 e). The low abundance during the LIA (B1) might be due to Atlantic waters being too cold during summers for this warm-loving species. (2003; figure 53 a) may have limited the reduction, or helped restart a stronger AMOC. The same pattern can be found in the Santa Barbara Basin (California), reflected in varve thickness variability, that is known to depend on annual precipitation, and inversely correlates with wind strength (Nederbragt & Thurow, 2005). Dark grey band corresponds to the 2000–3000 years band-pass filter of the data, with the light grey area the 90% confidence level. Temperature proxies at the West African sea indicate that SST were over 2° C lower during the African Humid Period (de Menocal et al., 2000; figures 40 & 55 e), after which the lack of precipitation due to the southward displacement of the African monsoon produced an abrupt warming of the sea surface before joining the global cooling trend of the Neoglacial.

An increase in salt deposition is associated with winter atmospheric conditions today. For the last millennia, the NAO intensity has also been reconstructed from lake sediments in Greenland, showing the very low NAO values that characterized the LIA (Olsen et al., 2012; figure 52 e). Andrews (2009) analyzed the distribution of foreign mineral species by drift ice in Icelandic shelf waters. The described ~ 2750-year cycle in varve thickness correlates very well with the Bray climate cycle (figure 54 e), with periods of higher varve thickness (increased precipitation) at the Bray lows. Within this complex general pattern, the lows of the Bray cycle are once more associated with a significant temporal reduction in SST (figure 55 e).

(1997), and at the LIA (B1) by Keigwin and Boyle (2000). During the early Holocene, the sub-thermocline was saltier, but underwent a freshening at a time when the ice sheets were still contributing meltwater. Such atmospheric processes are thought to explain the observed coupling between periods of excess drift ice delivery to Northern Iceland (Andrews, 2009; figure 53 c), and intervals of maximum inflow of warm Atlantic water to the Norwegian Sea (Giraudeau et al., 2010; figure 53 d) throughout the last 11,000 years. This is one of the reasons why it has been ignored for so long despite being present in multiple proxies and recognizable since 1912.

This means that all the lows in the Bray cycle had been identified as periods of reduced NADW contribution by different authors. The glacial freshwater discharge event of 8.2 kyr ago can be recognized. The hydrological 2400-year climate cycle Precipitation is affected by multiple factors, and in many cases determined by regional or even local climatic and weather patterns. Paleoclimatology has come to depend too much on the very reliable and precisely dated polar ice cores at the expense of the often contradictory, unreliable, and imprecisely dated climate proxies.

This is when the north polar vortex expands and meridional circulation increases, and thus represents an increase in cold and windy conditions. The evidence indicates a 2400-year periodic variation in SST and upwelling intensity off NW Africa that is associated with a climatic cycle in oceanic circulation that reflects periodic NAO conditions. While drift ice has been increasing in the past 6,000 years of Neoglacial conditions off Northern Iceland, the detrended data supports the existence of a 2400-year climatic periodicity. A high-resolution record of the strength of the Asian monsoon was obtained from oxygen isotopic analysis of stalagmite “DA” in Dongge Cave (China; Wang et al., 2005). Earth’s axis obliquity is shown to display a similar trend to Holocene temperatures. Holocene reconstruction of intermediate-water temperatures at 500 m depth from a suite of sediment cores in the Makassar Strait and Flores Sea in Indonesia, at the Indo-Pacific Warm Pool. A more complete analysis of SST temperatures in the tropical oceans and the North Atlantic region, the Mediterranean, and Red Sea, was performed by Rimbu et al. The principal mode of variability reflects Milankovitch forcing, delayed in the case of the North Atlantic by the melting of the ice sheets. The Bond record of drift-ice petrological deposition in the North Atlantic is also generally considered to correlate to colder conditions in the North Atlantic region that favor more frequent southward moving icebergs (Bond et al., 2001).

Significant reductions in C indicative of reduced NADW production have also been reported at 10,300 BP (B5) by Bond et al. Salinity reconstruction at the base of the thermocline by paired Mg/Ca–δ from a marine sediment core south of Iceland. Millennial-scale Holocene episodes of increased advection of heat by Atlantic waters off Norway are associated with enhanced winter precipitation over Scandinavia, increased sea-salt fluxes over Greenland, and strengthened wind over Iceland. This may be the location and intensity of the westerlies and the associated changes in mid- to high-latitude pressure gradients. Although the Bray climate cycle is present in the chemical record of Greenland ice cores, it is not easily seen or, maybe, absent in the Greenland and Antarctic ice core temperature records.

He observed in the data a possible 2300-2700-year cycle, that he projected into the past from the Little Ice Age, finding that a 2600-year period closely matched both vegetation transitions like the Atlantic/Sub-Boreal, or the Sub-Boreal/Sub-Atlantic transitions, and significant glacier re-advances from the past after the Younger Dryas (Bray, 1968). In this and following figures, blue bars mark the position of the lows of the ~ 2400-year Bray cycle. By the mid-70’s the scientific community was aware of the existence of a 2500-year climatic cycle that caused glacier advances and recessions, and that separated significantly different vegetation stages and cultural phases (figure 51B). In the negative phase, the polar low-pressure system (also known as the polar vortex) over the Arctic is weaker, which results in weaker upper level winds (the westerlies). Data is missing around the 8.2 kyr event when the basin entered a bioturbated non-varved interval similar to glacial stadials. The last 1300 years register a large increase in the frequency of floods in Spanish rivers.

Since he was the first to correctly identify and describe the ~ 2400 year climatic and solar cycles they should carry his name as this is the tradition. Due to its coincidence with C fluctuations, it was inferred that its cause was solar variability. Therefore, cold Arctic air and storm tracks move farther south, causing a drop in northern hemisphere temperatures and changes in precipitation patterns. Temperature and salinity analysis of the Atlantic Meridional Overturning Circulation (AMOC) using a sediment core south of Iceland, where the Faroe and Irmingen currents branch out of the North Atlantic current, shows that episodes of warm saline sub-thermocline conditions are centered at 0.3 (B1), 1.0, 2.7 (B2) and 5.0 (B3) kyr ago, coinciding with known climatic perturbations in the North Atlantic region (Thornalley et al., 2009; figure 53 b). The authors propose an increased preservation potential and/or increased human impact on the landscape as likely cause. That the global temperature reconstruction truly reflects global temperature changes and is not dominated by northern hemisphere records is confirmed by the Rosenthal et al.

Such periods might see a reduction in the northward flux of warm near surface waters in the North Atlantic to maintain mass balance (that could be the cause of the NADW reduction), and would result also in the cooling of North Atlantic high latitudes. Holocene North Atlantic and Arctic oceanic currents changes. Benthic C-rich North Atlantic deepwater (NADW) contribution. Warm saline sub-thermocline conditions took place at 0.3, 1.0, 2.7 and 5.0 kyr ago, coinciding with known climatic perturbations in the North Atlantic region. It is clear however that the atmospheric reorganizations that have accompanied the 2400-year Bray climate cycle are reflected in precipitation changes in several locations. Winter precipitation reconstruction at Bjørnbreen glacier in Jotunheimen, southern Norway. This has had the result that whatever is not prominently displayed in polar ice cores is considered unreal.

The lows in the Bray cycle (blue bars), correspond to periods of reduced NADW contribution. For decades Michael Magny has been studying Holocene mid-European lake level fluctuations and their impact on prehistoric human settlements (Magny et al., 2004). Precipitation is reconstructed from the known relation between variations in the equilibrium line altitude (ELA, the boundary between the ablation and accumulation areas) and mean July temperature variations reconstructed from palynological data. Another complicating factor is that the Bray cycle is not the only cause of climate change during the Holocene and thus proxies are full of signals whose origin is often difficult to ascertain, creating much confusion among researchers that results in contradictory reports. Conclusions 1) A 2600-year climate cycle was first proposed in the late 1960s by Roger Bray based on vegetation transitions and major glacier re-advances, and linked to solar activity.

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Andersson’s opposite view of continuous temperature evolution. Some botanists, like Rutger Sernander, proposed that these transitions were abrupt and not gradual. 5-year-resolution δO isotope record from Dongge Cave (southern China) stalagmite DA as a proxy for the strength of the Asian monsoon over the past 9000 years. A winter precipitation reconstruction from Norway’s coastal glaciers shows periods of increasing precipitation at the lows of the Bray cycle (Matthews et al., 2005; figure 54 b).