Bess G. Koffman | Search USGS

USGS2021

Glacial dust surpasses both volcanic ash and desert dust in its iron fertilization potential

The subarctic Pacific Ocean and Bering Sea comprise the second-largest high nitrate, low chlorophyll region in the world, where primary production is limited by the availability of iron (Fe). To estimate the potential impact of different terrestrial aerosol Fe sources on marine ecosystems, we performed a suite of laboratory assessments following established protocols, including: (1) leaching with Milli-Q water, (2) sequential extractions, (3) complete acid digestions, (4) X-ray diffraction, and (5) grain size analysis. Measurements were performed on 20 fine-grained (<5 μm) glacier-derived sediments from Alaska and the Yukon, two fresh, never-wetted volcanic ashes (Redoubt 2009 and Pavlof 2016), and six weathered ashes (Redoubt and Augustine) which span the past ~8,700 years. We compared results to published data on Asian desert-derived sediments, finding that the glacier-derived sediments have five times higher easily reducible Fe (median 2.3 ± 0.6 wt.%) than desert-derived samples (0.49 ± 0.1 wt.%) and fourteen times higher easily reducible Fe than fresh ash (0.16 ± 0.1 wt.%). In addition, fractional Fe solubility was higher in glacial sediment (median cumulative 0.31 ± 0.11% FeS) than volcanic ash (0.04 ± 0.02% FeS). Glacial sediments contained higher concentrations of other bioactive metals including Co, Ni, Cu, Zn, Mo, Cd, and Pb. Results suggest that glacier-derived dust may provide the subarctic Pacific with more bioavailable iron per unit mass than either volcanic ash or Asian desert-derived dust.

Alaska↗

USGS2015

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives 1 . Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa 2 , 3 , suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw 4 , 5 , 6 . Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events 7 , 8 , 9 . Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision 2 , 3 , 10 . Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2 σ ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

Nature↗