By Rod Chilton, http://bcclimate.com
The genesis of the Younger Dryas stadial (cold interval) remains an enigma. The onset was both climatologically unexpected and extremely sudden. The two principle theories are diametrically opposed and the proponents of both deeply committed. The debate to date has primarily been centered on some unusual “black mat’ deposits that may or may not be linked to a cosmic origin. What has been lacking in the wider discussion are all the other important features associated with the Younger Dryas. The following addresses many of these in hopes of their inclusion in future debates.
AND NOW THE REST OF THE STORY:
The Younger Dryas onset remains a little understood event. The cause of the 1,300 year-long interval continues to be debated. There are two completely different theories that have split the scientific community. One group strongly endorses an overall slowing or complete stoppage of the Northern Atlantic Ocean circulation 13,000 years ago. The other camp maintains that a catastrophic event originating from the cosmos was the cause.
Following on the heels of the mostly milder Bølling and Allerød intervals (interstadials), there was an extremely sudden and severe climate reversal. This was the Younger Dryas, first detected from Danish pollen studies as long ago as the mid 1930’s. Pollen from the Dryas flower, an arctic species, lends its name to this very cold interval. The Younger Dryas cold was first thought to have been confined to north-west Europe, with a possible extension to some other localities immediately surrounding the North Atlantic. More recently however, the cold climate shift is seen as world-wide in extent or nearly so.
The Younger Dryas appeared similar to earlier events known as Heinrich events that were prominent in the Pleistocene (approximately 70,000 to 14,000 years ago) (1). Their cause is not altogether clear, but marine cores, primarily in the north-east Atlantic are festooned with layers of sand, pebbles and rock (lithic materials). These deposits arrived in this area by being carried within “large armadas” of ice that upon melting deposited their lodes on the ocean bottom. Rapid climate shifts have been linked to ice melt from sea ice and the large continental glaciers that surrounded the North Atlantic. Lower salinity meltwater is less dense than ocean water and tends to float as a freshwater cap over the marine waters, and this is perceived as associated with North Atlantic Ocean circulation disruption. The Younger Dryas is understood to be linked with meltwater almost solely from the great continental ice sheets.
North Atlantic Ocean circulation has been likened to a great ribbon-like conveyor belt (2). Driven by temperature (thermo) and salinity (haline) differences, the thermohaline (THC) circulation is associated with the formation of North Atlantic Deep Water (NADW). The sinking of the NADW is alleged to result in the drawing north of warmer waters from southerly climes. This provides north-west Europe with its generally mild climate. However all of this is thought to change when the North Atlantic Ocean circulation is slowed or stopped.
The disruption of the THC approximately 13,000 years ago (BP) was first linked to meltwater originating from eastern North America’s Laurentide continental ice sheet (3). The bulk of meltwater just prior to the Younger Dryas had been flowing south via the Mississippi River, but as the Laurentide Ice Sheet retreated, an alternative and more conducive meltwater route opened. This was the St. Lawrence corridor, presumably a more favorable outflow path, that then caused North Atlantic Ocean circulation to slow or stop (4). As time has passed however, this idea has largely been abandoned. Not only did salinity levels in the offshore waters adjacent to the St. Lawrence maintain salinity levels similar to the present (5), but also the St. Lawrence corridor remained blocked by ice until well after the Younger Dryas ended (6).
Failure of the St. Lawrence River to deliver the melt has lead to alternative freshwater routes proposed. One of these involved the continent of Antarctica. The idea suggested here is that a significant increase in meltwater entry into world oceans took place approximately 14,300 to 14,600 years ago (7). An inundation known as “meltwater pulse – 1a” (mwp-1a) occurred with perhaps as much as 90% of the meltwater volume originating from Antarctica (8). Not only does this large freshwater flux cause the waters surrounding Antarctica to slow or shutdown, leading to cooling there, but also later, the eventual affect is felt within the North Atlantic. This is perceived to result in the two hemispheres as being out of phase, climatically. This has come to be known as the “bipolar seesaw,” with southern parts of globe warm while areas to the north experience cold. The opposite is often true as well, and this may have been the situation just prior to the Younger Dryas. As mentioned, eventually some scientists see the influence of the Antarctic melt reaching the North Atlantic. This is manifested as making the North Atlantic vulnerable to even small meltwater inundations (9). And is this critical threshold that many scientists see as occurring as the critical freshwater threshold was possibly realized approximately 13,000 BP (10). Not all researchers share this view, as at least one study assigned a much different date for mwp-1a, and that was shortly before 13,800 BP (11). And although these same researchers also conclude that the North Atlantic slowed or shutdown, the Antarctica as a significant meltwater source becomes questionable.
Since the Antarctic theory appeared, a number of other possible North Atlantic meltwater sources have been suggested. The first of these considered meltwater from the Laurentide as flowing northward through the Canadian Arctic via the Hudson Strait before reaching the North Atlantic (12). A second route was proposed more recently, and this was freshwater flowing across Arctic Canada from the main Laurentide source, across Lake Agassiz, then down the Mackenzie River and into the Arctic Ocean (13). The first of these meltwater corridors has now been shown to have remained blocked by ice throughout the early Younger Dryas, much like the St. Lawrence (14) and the second pathway, the Mackenzie, required adjustments to both the Laurentide Ice Sheet and the underlying landmass, before model simulations even allowed meltwater flow to take place in that direction (15).
As just mentioned, the main Laurentide meltwater source originated in the huge glacial lake, Agassiz. Most research has indicated that there was a significant lowering of the lake approximately 13,000 years ago. The assumption to date has been that most of the water exited by one corridor or another. However, recent research has determined that Lake Agassiz may not have experienced very much rapid outflow at all. Dr. Thomas Lowell of the University of Cincinnati contends that lake lowering resulted primarily from open lake evaporation when the lake was ice-free and some sublimation when it was frozen (16). However, this too has been disputed by another study that questions the very high rate of evaporation that the Lowell findings contend, this is a time when the climate was presumably very cold (17). The scientists that criticized the evaporation study however revert back to the now implausible explanation of the St. Lawrence meltwater route (18).
Certainly a very important question regarding the Younger Dryas is what effects, if any, were felt elsewhere in the world (away from the immediate confines of the North Atlantic). There are some indications that one outcome was similar to the most recent Heinrich event, specifically a warming of one to two degrees Celsius in the western tropical Atlantic and the Caribbean (19). The reason given for this warming is evidence of a response to strengthened easterly trade winds, which causes greater amounts of warm water to be driven into the Gulf of Mexico (20). Well to the west, on the north coast of South America the same stronger trade winds may also have induced ocean upwelling (21), that then lead to increased ocean productivity within the Cariaco Basin (22). However, the very premise of a trade wind induced warmer Caribbean and western tropical Atlantic during the Younger Dryas is now seen as suspect. Recent studies have shown that southeast portions of North America, the Caribbean and western tropical Atlantic all became much drier and colder at this time (23,24). Central America, for instance, shows a 300-400 metre lowering of the subalpine tree line. This is equivalent to a two to three degree Celsius temperature decrease (25). A number of other studies also indicate colder temperatures. One of the more revealing studies was undertaken within the Gulf of Mexico’s Orca Basin. The dependence of a number of marine organisms that respond differently to temperature and salinity variations is the basis for this interpretation.. One particular species Globigerinoides Ruber (tolerant of high salinity ocean water and cold ocean temperatures), when compared to five other marine species, less tolerant of cold and high salinity waters, depicted a sudden change in Orca Basin ecology some 13,000 BP (26). Originally, the Orca basin was thought to have become much more saline, the result of a sudden diversion of meltwater from the Mississippi to the St. Lawrence corridor. However, as previously shown the eastward meltwater route is now seen as implausible. Instead, it now appears that the Orca basin experienced a five or six degree drops in ocean temperature. (27). This has recently been confirmed by a new study that depicts significantly colder SST occurred within the Orca Basin (28). All of this is consistent with a meltwater pulse as continuing down the Mississippi corridor and not being diverted into the St. Lawrence pathway.
It is interesting to note too, that the Younger Dryas has been found to be a widespread event that extended well beyond the North Atlantic. The cold and predominately dry interval is now documented from all across North America and northward as far as Alaska. South America also experienced a definitive climate shift to a predominately cold and arid regime. This included the Amazon Basin, covering a significant portion of the tropical and subtropical latitudes of South America. Indications of an extreme drop in Amazon River levels to as little as 40% to 60% of present day levels are evidence of the drying (29). Lake Junin (11° S), a high elevation lake in the northern Andes is a second proxy showing an arid Younger Dryas, as lake levels were at their very lowest for the last 14,000 years (30). Not only did the climate become drier, indications are that it became colder too. Certainly the two to three degrees Celsius cooling in Colombia is an indicator (31). The aforementioned very low Amazon River level may well has been a response at least in part to decreased snowmelt and run-off from a colder Andes mountain chain. Further to the south in the Altiplano region (15° to 23° S), the climate during cold intervals like the Younger Dryas is expected to be wet (32). However, 13,000 years ago there appears to have been an exception (33). Indications are from the glacier Sajama (18° S) that a retreat of the glacier occurred, much as Glacier Quelccaya had done a little further to the north (34), (both likely responding to a colder and drier environment). Furthermore, considering once again the very low Amazon water levels, the Altiplano source region also appears to have been experiencing a decrease in precipitation.
Aside from a study from the Great Australian Bight (32° –35° S) (35) and an area near the edge of Antarctica (35) where distinct cooling was evident 13,000 years ago, the remainder of the Southern Hemisphere does not show definitive warming or cooling trends. Antarctica, at least the interior portions of the continent, may well be a different matter entirely. Here, the analysis of ice cores depicts a climate out of sync (bipolar seesaw) with the rest of the planet. Research suggests that very strong downslope (katabatic) winds prevent weather (climate) from penetrating any appreciable distance inland (36). However, it must be said that conclusions as to the Antarctic climate during the Younger Dryas are far from certain. There are problems having to do with the generally very light snowfall that is a feature of Antarctica. This prevents researchers from accurately differentiating climate intervals of less than about 2,000 years (37).
One type of methodology that permits past climate to be assessed depends upon the analysis of various gases that become trapped within ice after being deposited as snow within ice sheets throughout the world. The worldwide dispersion of most gases only takes one or two years, this allows comparisons of relative gas concentrations in localities as far apart as Greenland and Antarctica. The alignment of ice cores from low snowfall Antarctic and higher snowfall Greenland permits scientists to differentiate past climate. The problem is that it takes many years for the gas to be completely sealed off from the present day atmosphere. This varies between low snowfall areas like Vostok in Antarctica, where it takes as long as 2,500 to 6,000 years to “close off’ (depending upon the age of the ice deposit) to about 60 to 100 years in Greenland cores (38). The technique, while very good in determining the longer-term glacial and interglacial periods, at least in Antarctica is clearly inadequate for shorter-term events such as the Younger Dryas.
The continued contention that the North Atlantic was the principle trigger of the Younger Dryas has relied heavily upon a number of marine cores from the Atlantic. The first of these cores comes from the Bermuda Rise (EN120GGC1 – (33° 40’ N., 57° 37’ W)), where the analysis of benthic profiles of carbon 12 and 13 isotopes, along with cadmium/calcium ratios theoretically shows North Atlantic Ocean circulation disruption (39). However, a number of problems have been identified that relate to the Bermuda Rise marine core. Before analysis could be done a comparison was required with another marine core, CH73-139C (54° 30’ N., 16° 21’ W.), a core now found to have been affected by a condition called “bioturbation” (an unwanted mixing of the marine sedimentary layers) (40). This prevents precise dating as to the time when the slowing or stoppage of the ocean circulation occurred. (41). A second problem with samples from the Bermuda Rise is its location. Rather than sampling the desired amounts of deep water from the North Atlantic and Antarctic, it appears to be sampling an area where a localized mixing of ocean waters took place, which once again prevents accurate assessment (42).
The marine species Neogloboquadriana pacyderma, a polar organism displayed a definitive shift in population approximately 13,000 years ago, both at a marine core, Troll 3.1 (60° 47’N., 03° 43’W.), just west of Norway, and a second core V23-81 (54°02’N., 16° 08’ W.), just off Ireland’s west coast (43). Both of these studies have been drawn upon to support the slowing or complete shutdown of the North Atlantic Ocean circulation. A third study, that utilizes diatoms, (much more sensitive than Neogloboquadriana pacyderma), is very likely more appropriate in discerning relatively brief cold intervals such as the Younger Dryas (44). This study from the South-east Norwegian Sea does show a definitive shift of five to six degrees Celsius. However, this may not or may not be attributable to North Atlantic circulation disruption (45). The following quote highlights the researchers’ caution when they stated, “there is evidence that cooling was related to reduced salinities, but this does not prove a direct causal relationship that cooling was directly forced by meltwater events” (46). The shift instead may simply have been the result of the relative numbers of polar and arctic organisms (47). The inference drawn from this is that cold intervals such as the Younger Dryas may well have another altogether different trigger than North Atlantic Ocean circulation. Further to this, a somewhat more recent paper, also by the same researchers that conducted the study in the Norwegian Sea, indicates that a reduction in incoming solar radiation might be the trigger that initiates fluctuations in the polar front in the Nordic Seas (48). It is very intriguing that a reduction in incoming solar radiation may have occurred at a time when during the summer a maximum of solar energy should have been occurring (see ref 76).
The whole concept of North Atlantic Ocean circulation as having any appreciable influence upon the Younger Dryas is placed further in doubt by the work of Dr. Michael Sarnthein. Dr. Sarnthein has collected a large number of marine cores from throughout the Atlantic sampling the interval back to 30,000 before present (BP). The conclusion gleaned from his work reveals that the North Atlantic Ocean circulation was operative during the Younger Dryas, and had been so for more than 1,500 years prior to the start of this cold period (49). This is consistent with one other high-resolution marine core from the South Atlantic (presumably a very good location to detect North Atlantic Ocean circulation shifts) that also does not show a slowing or shutdown of the North Atlantic (50). Oceanographer Dr. Carl Wunsch has gone so far as to suggest that the whole concept of a temperature and salinity induced ocean circulation shift is in error, at least in the North Atlantic (51). Dr. Wunsch also believes that the North Atlantic is simply too small to cause significant climate changes in other parts of the world (52). Dr. Wunsch has even been more emphatic when he stated, “you can’t turn off the Gulf Stream as long as wind blows in the North Atlantic,” and then goes on to say that “the conveyor (the THC) is kind of a fairy tale for grown-ups” (53). . Dr. Richard Alley seems to echo these sentiments when he questioned how the small high latitude North Atlantic “energy starved polar tail” could possibly “wag the large energy rich tropical dog”(54). Apart from this, the presence of a less dense freshwater cap may not result in what many scientists see as a cooling at all. Instead, Dr. Richard Fairbanks sometime ago suggested that the presence of a shallow freshwater lid over more saline waters might be subject to rapid warming during the summer and early autumn (55). Thus, instead of the commonly perceived shift to cold associated with the presence of freshwater within the North Atlantic may well result in warming. This of course is the exact opposite of what many scientists currently believe occurred during the Younger Dryas. All of this presumes that there may have been less saline waters present in the North Atlantic during the Younger Dryas. According to many scientists this is not likely in the colder world of the Younger Dryas.
There are in addition a number of other perplexing factors apparent during the Younger Dryas: Carbon 14 (14C), for instance, increased markedly by 70% to 80% at the very beginning of the cold interval (56,57,58). This far exceeds the expected 30% or 35% 14C increase when the North Atlantic allegedly slows or shuts down (59,60). The consideration of possible 14C increases from geomagnetic changes or increased sea ice coverage are also thought to be quite insignificant (61). A second element, Beryllium 10 (10Be), also increased significantly approximately 13,000 years ago. Snowfall at this time in Antarctica and Greenland was much reduced, and it is this that some scientists see as the cause for higher 10Be concentrations (62). The contention is that the snow that did fall effectively removed beryllium from the atmosphere, thereby resulting in higher concentrations within ice. However, an alternative view is seen as plausible, and that is simply that there was much more 10Be in the atmosphere during the Younger Dryas (63,64). Both of these elemental forms are known to be products of cosmic events, and therefore lend credence to the Impact Hypothesis.
Two other deposits within Greenland and Antarctic glacial ice display interesting characteristics as well. Nitrates are one of these, and, though very difficult to analyze, there appears to be little doubt that much of the increase was attributable to very high amounts in the atmosphere (65,66). A second deposit, ammonium, was also greatly elevated during the Younger Dryas. The predominate origin for ammonia that arrives in Greenland is North America, and one reason proposed for very high levels is that biological activity remained very prominent because of a continuation of a mild climate (67). However, it is now known that North America did become significantly colder at this time, therefore making greater biological activity extremely unlikely. Thus, there are more questions than answers about the possible origins of the elevated levels of both nitrates and ammonium.
Even more intriguing, and more controversial as well, are a number of other deposits found both in soil and ice, possibly linked to a cosmic origin (68). Associated with an unusual “black mat” deposit found in many of the terrestrial sites, the dates for this layer are very close to the 13,000 BP Younger Dryas beginnings (69). What have garnered most of the attention thus far are items called “nanodiamonds and microspherules ,” produced under conditions of very high temperature and pressures (consistent with a cosmic origin). Scientists such as geologist Dr. Allen West contend that approximately 13,000 years ago “ a low density object” entered the Earth’s atmosphere, disintegrated explosively, and the remnants of the catastrophe rained down upon the planet (70). The signatures (including nanodiamonds) of this event are left behind throughout a widespread area that includes Europe, the Greenland Ice Sheet, North and South America, and now most recently from Central Mexico (71.72,73,74). Another very interesting recent development is the discovery of an approximately four-kilometre crater beneath the waters of the Gulf of St. Lawrence. This particularly discovery, it is suggested may be from most recent times, possibly from approximately 13,000 years ago (75).
Another perplexing feature of the Younger Drays is that it was a time of high solar insolation during the summer months. Solar receipt during the summer months when (somewhat surprisingly) is the most critical time for retention of snow and ice in the Northern Hemisphere and as a consequence an accumulation and growth of glacial ice (76). This particular alignment has occurred forty-two times over the past one million years and the Younger Dryas is noted as the only significant cold interval (77).
Two final features to be noted about the Younger Dryas, is that it took hold, not in decades as was once thought, but rather in as little time as a few years, or even less (78,79). This is but another piece of the puzzle that does not fit with the whole premise of an ocean induced short-term cold climate interval. It may be concluded that an alternative hypothesis, that of a very large cosmic event took place not that far from Earth, 13,000 years ago.
The evidence that supports this cosmic origin is available in much greater detail elsewhere, though a number of scientific papers are also referenced here.
Despite all of the preceding discussion as to its numerous shortcomings, the North Atlantic Ocean circulation as cause for the Younger Dryas remains the most widely accepted hypothesis. During the past several years, a debate has begun to swirl, as an alternative, a cosmic origin is gaining support.
This premise involves a possible impact or an airburst or disintegrating comet. To date, the primary focus in the attempt to justify a cosmic origin for the Younger Dryas has been almost totally limited to black mat deposits (specifically nanodiamonds and microspherules), detected in various parts of the world. This is far too limited an approach!
It is the purpose of this paper to attempt to raise the profile of the long list of other very important clues that also tell of a cosmic origin for the Younger Dryas. A list of these follows:
. 1) The North Atlantic Ocean circulation (known as the THC) slowing or shutdown was not triggered by meltwater suddenly shunted down the St. Lawrence, nor was it likely to have flowed north through Arctic Canada or have originated from the Antarctic.
2) Furthermore, dating of significant meltwater entries into the world’s oceans has not been contemporary with the Younger Dryas onset.
3) The main marine cores drawn upon as evidence for the THC hypothesis have either proven to be unreliable, or in some other cases only circumstantial.
4) And in contrast, with the just mentioned marine cores, are the proxies collected by Dr. Michael Sarnthein that depict the North Atlantic Ocean circulation as operative during the Younger Dryas and up to 1,500 years before the interval, as well as throughout the Younger Dryas, then continuing right on through the interval.
5) Increases of both 14C and 10Be are much too large to be associated with the North Atlantic alone.
6) Also, it is becoming increasingly evident that the onset of the Younger Dryas was indicative of atmospheric origins for the event, in that the onset was very rapid, perhaps in one year or less.
7) Finally, it should also be stated that such an extraordinarily severe long-lasting event occurred at a time when glacial and sea ice expansion took place, despite a peaking of Northern Hemisphere solar radiation in the most critical summer months.
Acknowledgements: My thanks to Steve Garcia and Clint Unwin for their valuable suggestions and thorough editing of the foregoing paper.
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