OVERVIEW
Soil and ice are two critical components of the Earth system when their functions are considered on a planetary scale. Soils, the life-sustaining layer of our planet that stores water, sequesters carbon, and releases nutrients, have fundamentally changed the geomorphic, hydrological, and biogeochemical functioning of Earth’s surface making it distinctly different from other terrestrial planets and Earth’s own rocky origin. Soils have laid the foundation, both literally and figuratively, for human society and our civilization. Ice, in which over 68% of Earth’s surface fresh water is stored, is a dominant player in regulating the global climate and sea level. Ice sheets and glaciers, though remote and out of sight for most of the population on Earth, have an outsized and far-reaching impact on people’s lives and our collective future in the context of ongoing climate change on this planet.
With dual training in geomorphology and glaciology, my current research interests and experience are focused on quantifying physical and chemical processes operating in Earth’s Critical Zone that produce soil and shape topography, and modeling dynamical processes of ice sheets and glaciers in response to climatic and non-climatic drivers in Earth’s cryosphere. I explore how climate influences soil production and soil chemical weathering, how erosion drives landscape evolution through drainage divide migration and drainage basin reorganization, and how rapid water level changes in proglacial lakes affect grounding line dynamics of water-terminating ice sheets and glaciers. I am driven to achieve a mechanistic understanding in the physical basis of these seemingly disparate sub-systems of Earth’s dynamic surface through a combination of field observation and measurements, laboratory analysis, and mathematical modeling.
Simulation of the dynamical response of a deepwater-terminating ice sheet to a rapid lake level drop.
The rich spectrum of interacting physical processes between ice and water at galcier and ice sheet margins. From Carrivick et al. (2020).
Today proglacial lakes are ubiquitous at glacial termini worldwide, and the ice-water interaction represents one of the largest uncertainties in predicting the glacial and ice sheet behaviors and their contribution to sea level changes. My current work provides an ideal launching point and exciting opportunities to systematically monitor targeted physical processes at the lake – ice boundary at modern analogues and improve the representation of the complex ice-lake interactions in numerical models to help elucidate glacier and ice sheet behaviors in the past and future.
Dynamics of lake-terminating glaciers and ice sheet
Proglacial lakes fringing a continental ice sheet turn sections of its terrestrial margin into water-terminating boundaries analogous to those in marine-terminating settings. This change of boundary condition makes the land-terminating ice sheet margin subject to a rich but complex spectrum of physical processes characterizing ice-water interactions, which has been widely investigated in marine-terminating glaciers and ice sheets. How do ice-lake interactions at the terrestrial margin of a continental ice sheet affect ice flow dynamics, thereby impacting the mass balance and geometry for the entire ice sheet? The investigation into this question has been rare so far.
The Superior Lobe of the Laurentide Ice Sheet (LIS), which occupied and flowed through the modern-day Lake Superior basin, was a lake-terminating outlet glacier draining the southern margin of the Laurentide Ice Sheet. Its deglaciation history at the end of the Last Glacial Maximum appears non-monotonic in the geological records, with evidence for multiple re-advances in the warming climate. Paleo-records also suggest that the lake level in the Lake Superior Basin was subject to large and fast changes during the last deglaciation when the retreating ice sheet margin successively opened spillways that rapidly drained the lake. My current work investigates the interplay of water-terminating ice sheet dynamics, warming climate conditions, and rapid lake level fluctuations of the Lake Superior Lobe (Hu & Haseloff, to be submitted to Geology). The results from my work show that rapid water level changes in the proglacial lake can trigger highly transient grounding line and ice sheet behaviors. My research provides a non-climatic feedback mechanism that drove the highly irregular margin fluctuations of the Superior Lobe which were asynchronous with the climate during deglaciation.
How sensitive is silicate weathering to climate?
The sensitivity of silicate weathering to climate is of wide interest because it affects ecosystem nutrient fluxes, Earth’s topographic evolution, and the stability of Earth’s long-term climate. Despite considerable progress in understanding how climate variables affect mineral dissolution rates in theory and in well-controlled laboratory experiments, the climatic control on chemical weathering in nature remains elusive.
To better constrain the sensitivity of silicate weathering to climate, I measured soil chemical depletion and chemical erosion rates along a steep elevation gradient that spans nearly 3 km in altitude and ~18 ℃ in mean annual air temperature on the granitic San Jacinto Peak (SJP) in southern California. Here I've deployed in-situ climate monitoring stations (picture on the right) over 3 years during my PhD to measure soil temperature, soil moisture, air temperature and precipitation at 15-minute intervals along a steep elevation transect at SJP. To quantify the influence of dust on the soil chemistry, I teamed up with Prof. Sarah Aarons' group at Scripps Institution of Oceanography, UCSD to measure dust flux and composition at 6 of the instrumented sites. Our work at SJP shows dust is chemically distinct from the local bedrock, and dust deposition represents a secondary but non-negligible contributor to soil composition and mass, which highlights the importance of accounting for dust contribution in investigations of soil chemical weathering. Together, my PhD work motivates further investigation into teleconnections between dust sources, nutrient supply, climate seasonality, and chemical weathering in mountain ecosystems. Stay tuned for publications that will soon come out from our work at San Jacinto Mountain!!
Dr. Ken Ferrier (left) and Kai were setting up a passive dust collector at site SJP483. The snow-covered peak in the background is the San Jacinto Peak (3283m).
Large discontinuities in channel head 𝜒 values across the drainage divide between the headwater catchment of Hei River and its "aggressive" neighbors.
Kai was collecting sediment from an active channel in the Qilian Shan for Be-10 measurement.
How fast do drainage divides migrate and drainage basin re-organize?
Drainage divides, backbones of terrestrial landscape, are dynamic features that migrate across Earth's surface. The resulting changes in river networks hold important implications for sediment and nutrient transport across Earth's surface and the geographic connectivity between biotic species.
Divide migration is ultimately driven by differential erosion across the divide. In the Qilian Shan, I documented a near-exponential relationship between cross-divide differences in erosion rates (ΔE), which I measured with cosmogenic Be-10 in stream sediment, and channel-head values of the topographic metric 𝜒 (Δ𝜒). This provides empirical support for the hypothesis that Δ𝜒 can reflect ΔE, which is useful because Δ𝜒 can be measured straightforwardly with topographic data. The exponential relationship in the Qilian Shan is similar to those found in the Southern Appalachians and Ozark dome by Willett et al. (2014) and Beeson et al. (2017), which suggests a common set of processes driving divide migration across diverse mountain ranges. In a simple numeric model, the near-exponential ΔE-Δ𝜒 relationship naturally arises as asymmetric topographic divides approach equilibrium. To learn more about this research, please read Hu et al. 2021. EPSL.
Beyond my work on the divide migration in the Qilian Shan, I am interested in further testing the global prevalence of the near-exponential ΔE-Δ𝜒 relationship by building a database of differential erosion rates and channel-head 𝜒 values across actively migrating divides. I am also interested in exploring the mechanisms of drainage basin organization in more detail, especially the potential feedbacks between divide migration, hillslope erosion, soil chemical weathering and basin hydrology.