(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.
By Javier The existence of a ~ 2400-year climate cycle, discovered in 1968 by Roger Bray, is supported by abundant evidence from vegetation changes, glacier re-advances, atmospheric changes reflected in alterations in wind patterns, oceanic temperature and salinity changes, drift ice abundance, and changes in precipitation and temperature.
This is established with proxy records from many parts of the world.
The latest three periods correspond with Bray lows 2 to 4 (figure 53 a). Five peaks in residuals from the data are defined by the 2500-year cycle. Abundance of coccolith species in a marine core off Norway reflects major Holocene changes in Atlantic water transfer to the Nordic Seas with a 2400-year periodicity (Giraudeau et al., 2010; figure 53 d). Even B5, when the world was still experiencing the fast warming that led to the HCO, shows a significant departure from the warming trend of the previous centuries. This conclusion agrees well with the other evidence shown here for the Bray climate cycle. Holocene millennial-scale sea-surface temperature variability. The dominant modes of tropical and North Atlantic Holocene SST display a 2.3 kyr cycle linked to the strength of AO/NAO during the Holocene, showing that this cycle has a global character. It is the most important climatic cycle of the Holocene.
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.
His research shows very clearly the impact of Holocene climatic change. Winter precipitation is more important than summer temperature for glacier expansion episodes. 2) This climate cycle is clearly evident in numerous proxies from the North Atlantic region and other places in the world that reflect ~ 2400 year-periodic changes in wind patterns, oceanic currents strength and salinity, drift ice, precipitation, and temperatures.
There is a general trend to increasing dryness during the Neoglacial, after a wetter HCO. Holocene mid-European lake-level reconstruction from a data set of 180 radiocarbon, tree-ring and archaeological dates of higher and lower lake-level events based on multiple lines of evidence, obtained from sediment sequences of 26 lakes in the Jura mountains, the northern French Pre-Alps and the Swiss Plateau. Acknowledgements I thank Andy May for reading the manuscript and improving its English.He also constructed an index for postglacial major ice re-advances from glaciers all over the world. Holocene subdivisions and glacier fluctuations in the European Alps showing the complex pattern of advances and retreats that do not always correspond between Austrian and Swiss Alps. Since then glaciologists have reconstructed Holocene glacier movements from hundreds of glaciers all over the planet, and glacier variability has become more complex (figure 51 B). Light grey trace, inferred NAO circulation pattern from the principal component analysis of redox parameters (Ca/Ti and Mn/Fe ratios) from a Greenland lake sediment record. The changes in polar circulation and wind strength are suggestive of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). 25-year average sedimentary varve thickness record at a marine core in the Santa Barbara Basin as a proxy for annual rainfall in the area. In addition, B3 and B4 lows are also characterized by significant episodes of slackwater floods or paleofloods, that record periods of increased flood frequency related to Holocene climatic variability (Thorndycraft & Benito, 2006; figure 54 c).He compared these two observations and found a high degree of correlation, and good agreement with Icelandic sea-ice, and C production variations. The uncalibrated radiocarbon dates scale is shown together with the corresponding calibrated scale in calendar years BP. Today we still recognize the major global advances that define the 2400-year cycle (Mayewski et al., 2004; figure 47), but there is hardly a century, especially during the Neoglacial, when glaciers were not advancing somewhere. The AO reflects the state of the atmospheric circulation over the Arctic, through a positive phase, featuring below average geopotential heights, and a negative phase in which the opposite is true. Thin line represents lowpass filter to emphasize millennial scale fluctuations. They are fine-grained sediments produced by large magnitude floods, preserved in valley side rock shelters in bedrock gorge reaches.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.Even then, they were thought to represent variations in solar activity. Paul Mayewski, one of George Denton’s students, was the scientist in charge of coordinating the effort of over 200 scientists in the American Ice Core Program that in 1993 completed the Greenland Ice Sheet Project II (GISP2). A NAO negative phase is produced when the weakening of the Iceland low and the Azores high reduces the pressure gradient resulting in weaker more southern westerlies producing colder conditions over much of North America and Northern Europe while moving the storm tracks southward towards the Mediterranean. They propose solar variability as the forcing behind these oscillations. Under humid conditions trees were unable to grow on wetter bogs. Although Ireland hydrology shows a complex pattern over the increasingly wet Neoglacial trend, lows in the Bray cycle are associated with periods of increased precipitation (figure 54 d). Superimposed on this trend are millennial-scale SST variations coherent with some of the North Atlantic ice-rafting events defined by Bond et al. (2013), and shows that every Bray cycle low coincides with a significant downward departure from the general temperature trend (figure 55 c).