Southern Hemisphere Dust

My dissertation research (2009-2013) explored the potential of the Southern Kalahari as a future dust source. Our primary motivation was driven by earlier studies in this region where they showed a relationship between threshold grass cover and aeolian activity in this region and that increased aeolian activity was observed during periods of extended drought. Similarly, in other dryland regions around the world, the effect of landuse change on aeolian activity is well established. Study of aeolian dust from the Southern Hemisphere sources is especially important because, 1) most sources of dust are found in the Northern hemisphere and, 2) the proximity of these dust sources to the Southern Ocean play an important role in ocean biogeochemistry.

My research showed that interdunes within a precipitation belt that receive less than 250 mm mean annual prepciation can emit dust similar to the other currently active sources in the regions such as the Makgadikgadi Pan and Etosha Pan (Bhattachan et al., 2012). An interesting piece of the puzzle then was to determine where this emitted dust will go. We found that 26% of the trajectories (simulated with a trajectory model, HYSPLIT) reaches the Southern Ocean which is a High Nutrient Low Chlorophyll (HNLC) region where soluble iron (Fe(II)) is a limiting micronutrient. We found that sediments from the interdunes are rich in soluble iron and could play an important role in ocean biogeochemical cycles. We calculated that 0.42 Tg/yr of dust could be emitted from the region under a scenario of 100% vegetation loss (Bhattachan et al., 2013). In addition to the environmental impacts, the reactivation of dunes also has socioeconomic impacts in the Southern Kalahari; for example, loss of palatable grass and decline in rangeland productivity pose a serious threat to the commercial interests and people’s livelihood in the region. One of my disseration chapters focused on the resilience of the duneland vegetation in the overgrazed areas of the southern Kalahari. To this end, we used field observations and soil seed bank experiment to show that palatable perennial grass recovered in dunes after exclusion of grazers (Bhattachan et al., 2014). In systems where high resolution continuous data of variables undergoing state changes/regime shifts are not available, we showed that ecological indicators such as changes in percent grass cover, grass community composition, and seed bank could serve as effective early warning indicators of state shift.

In my role as a postdoctoral research associate at UVA (2013-2015), I continued to expand the analysis carried out in the Southern Kalahari to other dust hotspots in the Southern Hemisphere. We showed that agricultural expansion in the Mallee region in Australia could emit dust rich in soluble iron and is likely to be deposited in the Southern Ocean (Bhattachan and D’Odorico, 2014). During Isabel Reche’s visit to UVA, we discussed about using their samples collected in aerosol samplers in the Sierra Nevada mountains in Spain to quantify the enrichment of soluble iron during atmospheric transport from North African dust sources which previously only relied on modeling studies (Bhattachan et al., 2016).

In a recent study, we analyze the likely depositional footprint of dust emission from continental sources in the Southern Hemisphere and calculate how much of it will likely to be deposited in the Southern Ocean (Talthego et al., 2020). A synthesis paper currently in preparation seeks to answer how much carbon would be sequestered via ocean fertilization from the soluble iron contained in the Kalahari dust.