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WEaThering in bedrock landSLIDE deposits

Periodic Reporting for period 1 - WetSlide (WEaThering in bedrock landSLIDE deposits)

Reporting period: 2019-10-01 to 2021-09-30

Understanding Earth’s climate system is a key scientific challenge of the 21st century. It is a basis to climate change mitigation and adaptation and underpins, for example, the study of Earth surface dynamics, the evolution of life, and the sedimentary rock record. Earth’s climate is linked to the concentration of carbon dioxide (CO2) in the atmosphere and, thus, to the global carbon cycle. An important component of the carbon cycle is the chemical dissociation (weathering) of rocks near Earth’s surface. Where silicate minerals are exposed to the surface of the Earth by uplift and erosion, they weather and drive the precipitation of carbonate minerals. The carbonates lock up carbon in the rock record over hundreds of millions of years.

Rates of weathering reactions are controlled by the availability of weatherable bedrock. Uplifting mountain ranges expose >50% of the global rock mass. Therefore, they are hotspots for chemical weathering and could dramatically alter Earth’s climate. However, our understanding of the link between exposure of rock by erosion and their chemical weathering is limited by three main knowledge gaps (KG).

KG1: Existing datasets that link erosion and chemical weathering fluxes are characterized by a co-variation between erosion rate and climate. Thus, it remains difficult to unravel the relative importance on chemical weathering of climate and the exposure of fresh minerals.

KG2: Weathering rates and their effect on atmospheric CO2 depend strongly on mineralogy. Whereas silicate weathering with carbonic acid sequesters CO2, the oxidation of sulfides (such as pyrite) coupled to carbonate dissolution releases CO2 to the atmosphere. The effect of different lithologies on chemical weathering is poorly understood.

KG3: Weathering models are based on the continuous chemical alteration of bedrock and the formation of soils. However, as erosion rates increase, stochastic landslides dominate the exposure of rock and may strongly influence weathering fluxes. However, no models and very few data exist that explore chemical weathering of landslide deposits.

The proposed aim of WetSlide was to address KG3 with newly acquired data from New Zealand. However, a re-analysis of the literature and the covid pandemic led us to address KG1-3 (see below). This change affected the order of scientific investigations, but the dissemination, communication, mentoring, career development, and training goals were largely met as planned.
The work followed three research objectives

RO1: Analyze the dependence of silicate, carbonate, and sulfide weathering on erosion rate.
We analyzed dissolved solutes in water samples from rivers in southern Taiwan and along the eastern margin of the Tibetan plateau (collected prior to the action) and found that silicate weathering rates do not increase with exposure of rock by erosion. Instead, coupled sulfide and carbonate weathering rates increase and may retard silicate weathering. As a result, CO2 emission at high erosion rates is more than twice as efficient as CO2 sequestration in more slowly eroding portions of the orogen. Despite different lithologies, data from Taiwan and the Tibetan Plateau revealed similar patterns. Thus, our results could be applicable to metasedimentary and felsic lithologies globally. These results were disseminated through a publication in Nature Geosciences and a manuscript submitted to Earth Surface Dynamics.

RO2: Assess the impact of landslides on the link between erosion and weathering.
We compiled weathering data from mountain ranges across gradients of erosion and landsliding and found a remarkable similarity in all datasets. We suggest that the onset of landsliding may cause minerals to bypass the weathering zone. Could landslides act to weaken the relationship between erosion and weathering rather than strengthen it? An associated manuscript is in preparation.

RO3: Assess the impact of landslide deposits on landscape-wide weathering rates.
We sampled river waters as well as groundwater seeping out of landslide deposits in New Zealand. Preliminary analyses suggest that solute concentrations in waters from recent landslide bodies are elevated with respect to the regional average (Fig. 2). However, concentrations decline with deposit age, and become similar to the regional background within a few tens of years. It is therefore possible that landsliding may not significantly boost chemical weathering. The main effect of landslides on the carbon cycle may instead be the export of organic carbon (plants and soils) and the rapid re-grows of that carbon stock, a question that we investigate with a master’s thesis. Dissemination of the data from New Zealand is in preparation.

Collaborative research
In addition to above work, I co-advised a PhD student at ETH Zurich, Dr. Erica Erlanger (now at GFZ), and continued the collaboration into her postdoc. Her work focuses on links between chemical weathering and erosion in the Apennines. A first study was published in the Journal of Geophysical Research. I also helped Dr. Roda Boluda (VU Amsterdam) at measuring erosion rates along the western Southern Alps of New Zealand. Her new data suggest that temperature-controlled peri- and paraglacial erosional processes dominate denudation and a first manuscript was submitted to Nature.

Throughout the action, work was disseminated at six conferences, during five seminars at international institutions, and through blog posts and press releases.
A key finding of this work is the insensitivity of silicate weathering to erosion rates in a number of mountain regions. Instead, erosion appears to boost reactions that emit CO2, such as the weathering of carbonates and sulfides. These findings demand a re-evaluation of carbon cycle models that assume a positive relationship between erosion silicate weathering, and CO2 drawdown. We confirm that fresh landslide deposits are efficient weathering reactors. However, as deposits age, the increased weathering efficiency is rapidly lost. Instead, preliminary analyses suggest that landslides may cause rocks to bypass the weathering zone and weaken the relationship between erosion and weathering.

The action so far yielded two published manuscripts, two submitted manuscripts, one manuscript in preparation, and additional results that will be prepared for publication within the next year.

In addition to the scientific impacts, the MSCA action played a significant role in career development of myself as well as a master’s student. I have become competitive for assistant professorship jobs and have been given job interviews at Oxford University and at the Ludwig Maximilian University of Munich. Further, during the spring and summer of 2021, I developed and submitted a research grant for a junior research group. Following review of the proposal by the German Science Foundation, I recently advanced to the interview stage of the proposal evaluation.

Finally, in 2019, I co-founded an employee initiative with the aim to unite efforts in reducing CO2 emissions attributable to our research within the Helmholtz Association - the largest research association in Germany. Our work has accelerated steps toward emission-reductions and a commitment of the board of directors to sustainable research. These efforts have been my most formative administrative experience that I anticipate to build on in my future career.
Weathering in Landslide deposits