Determining the imprint of Heinrich Stadial 4 on the latitudinal distribution of methane sources using the inter-polar methane difference from the WAIS Divide and GISP2 ice-cores
Abstract:Ice core records of atmospheric methane show that on orbital and millennial timescales methane closely follows changes in Greenland temperature, presumably driven by variations in tropical hydrology. Over glacial-interglacial cycles methane mole fractions can range from 350ppb up to 700ppb. Over the last 110,000 years, warm events referred to as Greenland Interstadials (GI), noted in Greenland ice core δ18O records as well as in ocean sediments, speleothems, and other archives, are characterized by a coeval rise in atmospheric methane (Brook, Harder et al. 2000). At the onset of these events methane rises by 50 to 300ppb from Greenland Stadial (GS) concentrations (Brook, Sowers et al. 1996). In addition to the GI/GS methane variations, there are distinct, though smaller, increases in methane during Heinrich Stadials (HS) 1, 2, 4 and 5(Rhodes, Brook et al. 2015). These HS features in the methane record are unique as there is no coeval change in Greenland water isotopes or in 30°N summertime insolation.
Using the Inter-Polar Difference (IPD) of methane mole fractions, we investigate the hypothesis that the Inter-Tropical Convergence Zone (ITCZ), normally positioned north of the equator, shifted southward during HSs, causing an intensification of monsoons and tropical wetland methanogenisis in the southern hemisphere and drying in the northern hemisphere, as suggested by cave records in China (Wang, Cheng et al. 2001) and Peru (Kanner, Burns et al. 2012). For this study we obtained a high-resolution, high-precision methane record from the WAIS Divide and GISP2 ice cores covering GS 10 through GI 8. Because methane is sufficiently well mixed in the atmosphere to allow synchronization of Greenland and Antarctic ice cores, yet has a short enough lifetime to preserve an inter-hemispheric gradient, a change in the latitudinal distribution of sources can be determined from the difference between these two records. Our data show a distinct decrease in the gradient over the first 500 years of HS4, reaching a minimum proximal to the mid-stadial rise in methane. The gradient then gradually increased over the remaining 1000 years of the stadial. This shape in the gradient suggests the maximum southward migration of the ITCZ occurred mid-stadial and then returned northward for the remainder of the stadial.