The Influence of Glacial Cover on Riverine Geochemistry in Chilean Patagonia

Abstract

Glacial environments are biogeochemically active and interact with the climate system via feedback processes affecting weathering, nutrient export and primary productivity. However, most previous studies have focused on the polar ice sheets, and it is unclear how these processes vary across a mid-latitude deglaciating landscape. This thesis provides the first regional-scale assessment of how ongoing glacial retreat affects silicon (Si) and iron (Fe) exports, and Si isotope compositions in 40 rivers in Chilean Patagonia, enabling projections about the biogeochemical impacts of climate change across this region. First, I investigate how variable glacial cover impacts riverine concentrations, size-partitioning and yields of bioessential Si and Fe. I show that dissolved (0.45 µm) Si and Fe are positively correlated with glacial cover, suggesting that export of these phases is likely to decrease with continued glacial retreat. Next, I examine how variable glacial cover impacts riverine Si isotope compositions. A strong linear correlation was observed between 0.45 µm filtered Si isotope compositions and upstream glacial cover, reflecting the inclusion of isotopically-light colloidal-nanoparticulate feldspars within the filtrate. I also show how amorphous particulate (>0.45 µm) Si phases impact riverine Si compositions and should be included in terrestrial isotope budgets. This research demonstrates the importance of size-fractionated analyses and the need to constrain the potential lability of different phases to interpret Si isotope cycling. Finally, I investigate how different filtering methods affect the measured concentrations of riverine Si, Fe and other trace metals. Large differences in elemental concentrations were observed between filtrates, especially in glacier-fed rivers where high suspended sediment and colloidal-nanoparticulate loads accentuate filtering artefacts. The offsets between filtering methods have significant implications for calculating weathering budgets, Si isotope compositions and nutrient exports in this region, highlighting the critical need to develop standardised sampling methods for rivers. Overall, this thesis furthers our knowledge of how glacial processes affect riverine elemental concentrations and isotopic compositions, helping to understand the global-scale ramifications of continued deglaciation in a warming world.

Date of Award	23 Mar 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jemma L Wadham (Supervisor), Laura F Robinson (Supervisor), Katharine Hendry (Supervisor) & Jon R Hawkings (Supervisor)