Morrill, Carrie. "Palaeoclimate: Asian Connections." Nature Geosci 6, no. 2 (02//print 2013): 91-92.

Australia: The Land Where Time Began

Asian Connections

The Northern Hemisphere cooled about 8,200 years ago (8.2kyr Event) when the overturning circulation of the Atlantic (AMOC) slowed, which has been shown by a speleothem record from China, to indicate it coincided with a period of drying as identified in the ice cores from Greenland.

According to the author¹ a possible consequence of continued greenhouse warming is for the Atlantic Meridional Overturning Circulation to slow (IPCC report, 2007). A northward flow of surface waters that is relatively warm and salty in the North Atlantic characterises the overturning circulation, these waters eventually sinking to depth where it flows towards the equator, redistributing heat from the tropics to the region of the North Atlantic. This circulation can be disrupted by large inputs of freshwater to the surface waters of the North Atlantic, such as from melting ice sheets. Throughout the last glacial period there are periods during which the overturning meridional circulation weakened as a result of freshening of surface waters. The climate across the Northern Hemisphere was affected during periods in which the circulation was weakened, in particular the Asian monsoon system dynamics. It has been shown by Liu et al., (2013) in a paper in Nature Geoscience that a slowing of the AMOC in the early Holocene interglacial 8,200 BP resulted in a reduction of monsoon precipitation in east Asia, though the period leading up to the 8,200 event was relatively warm.

Large volumes of freshwater entered Hudson Bay and the North Atlantic during the 8.2kyr Event, raising the global sea-level by 1.5-3.0 m. (Tornqvist, 2012). The water was released by the catastrophic drainage of Lake Agassiz, a proglacial lake on the southern border of the ice sheet that had previously covered North America, as well as possibly an ice dome over Hudson Bay (Tornqvist, 2012; Gregoire et al., 2012), but which was retreating at this time. The exact route by which the water from Hudson Bay reached sites in the North Atlantic deep water formation remains to be agreed upon (Ellison et al., 2006), but according to the author¹ there is increasing evidence indicating a weakening of the AMOC at this time (Kleiven et al., 2008).

It is shown by proxy records that the 8.2 kyr Event had a global footprint with alterations of the hydrological cycle and cooling in the Northern Hemisphere, though it is less clear what the effects on Asian precipitation was at this time. Rain that seeps into the ground to form speleothem deposits in caves that capture the chemical composition of the rain provide some of the best moisture records. Analysis of speleothems from China has found them to have oxygen isotope ratios that have been interpreted as indicating a reduction of the precipitation from the summer monsoon during the 8.2 kyr Event (Cheng et al., 2009). Because of the possibility of the isotope ratio variations being caused by changes in the mixtures of the rain from summer and winter, or possibly in the source region of the monsoon moisture in the ocean (LeGrande et al., 2009; Pausata et al., 2011).

This problem has been addressed by the use of high-resolution geochemical measurements from Heshang Cave, China (Liu et al., 2013). In their study, the oxygen isotope values of the carbonate in the speleothem were compared with the ratio of magnesium/calcium in the same carbonates. The Mg/Ca has been found to be predominantly sensitive to the amount of rain that falls over the time the carbonate is precipitated (Fairchild & Treble, 2009). More calcite precipitation occurs from groundwater before these waters reach the cave at times when the water flow rate of the aquifer above the cave is low. Calcium precipitates from water more easily than magnesium, leading to karst waters precipitating more calcium during times of low flow rates before reaching the speleothem resulting in the speleothem having higher Mg/Ca ratios. The Mg/Ca proxy provides a strong signal of the amount of rainfall in the region of the Heshang Cave, though it is not quantitative.

The Mg/Ca ratios are higher in the portion of the Heshang speleothem that covers the period of the 8.2 kyr Event, and are higher than at any other time in the record of the Holocene. There is also a high level of correspondence of the period of high Mg/Ca ratios with that of high oxygen isotope values, which are also believed to indicate lower levels of precipitation. Robust evidence is provided by these results for reduced precipitation during the 8.2 kyr Event at this site that is not rivalled over the past 9,000 years. A relative chronology has been developed, based on high resolution sampling and annual banding in the stalagmite, for the variations that occur from year to year that is constrained tightly by U-Th dating. The author¹ suggests there are not many records of the 8.2 kyr Event that are constrained as tightly, the most notable being the Greenland ice core records that are annually resolved (Thomas et al., 2011). It is shown by these records that the period covered by the record of the 8.2 kyr Event was about 160 years, and the period of the most extreme portion of it being 69 years. Using the same statistical approach, analysis of the Heshang time series has yielded a period of about 150 years for the dry period in Asia, and the maximum anomaly lasting 70 years. The temporal evolution of the 8.2 kyr Event in east Asia and Greenland is virtually identical, when allowance is made for noise in the proxy records and the small uncertainties in layer counting.

A tight coupling of North Atlantic cooling and weakening of the monsoon is indicated by the remarkable similarities displayed for the 8.2 kyr Event record in both Heshang and Greenland. Morrill suggests the most important point is that a teleconnection between the 2 regions operates on an annual scale with no significant smoothing or lag time. The teleconnection is implied by its rapidity of response to be atmospheric, and not oceanic, Morrill suggesting it may result from high latitude planetary reflectance. If more snow cover on land or a greater extent of sea ice, characterise cool periods in the North Atlantic, increased levels of solar radiation will be reflected to space from the surface of the Northern Hemisphere. The intertropical convergence zone shifts further south, towards the warmer hemisphere, because of interhemisphaeric temperature asymmetry that is associated with extreme cooling in the Northern Hemisphere. Tropical rainfall is reduced in the Northern Hemisphere as this shift occurs. As the monsoon is fuelled by the regional land-to-ocean temperature gradient enhanced cooling of the land surface of Eurasia could reduce the intensity of the monsoon

It has been demonstrated that there is a connection on annual scales between precipitation in China and the AMOC (Liu & Dong, 2008), that is maintained even during interglacial climatic conditions that are relatively warm. To determine if this connection holds for a future warmer climate the nature of this link needs to be further studied.

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