- Webb Hall 1100
Abby Wyant and Alexander Neely, both UCSB Earth Science graduate students, will each give a talk at Speakers Club this Thursday. The title of Abby Wyant's talk is "Did Increased Hydrothermal Activity Trigger The Convergent Evolution Of Siliceous Skeletons In The Cretaceous?" The title of Alexander Neely's talk is "An Automated System of Stream Knickpoint Definition and Extraction."
Abby Wyant's abstract:
Silicon biomineralization is first seen in the Cambrian with the appearances of siliceous sponges and polycystinean radiolarians, yet most siliceous taxa are not seen until the Cretaceous, almost 400 million years later. Bioavailable seawater silica is known to affect silicon biomineralizers in various ways, controlling radiolarian skeletal robustness, diatom reproduction, and sponge spicule morphology. The silicon cycle of the Cretaceous is explored to determine if silicic acid concentrations were heightened, favoring the acquisition of a siliceous skeleton. The greatest source of dissolved silicon to the modern ocean is from riverine input, though low 87Sr/86Sr and Mg/Ca values, caused by the widespread emplacement of oceanic plateaus and the enhanced production of young oceanic crust during the Cretaceous, indicate hydrothermal sources may have dominated the silicon budget at this time. The contributions of silicon from oceanic plateaus and mid ocean ridges are considered; it is possible mid ocean ridges may have been a significant contributor to the dissolved silicon flux of Cretaceous marine waters. Future research aims to construct a three component mixing model of continental runoff, hot springs, and seafloor weathering using 87Sr/86Sr and Mg/Ca data to better constrain the Cretaceous silicon budget.
Alexander Neely's abstract:
Many knickpoints (or stream profile convexities) move as migratory kinematic waves stemming from rapid baselevel fall events, usually induced by factors like relative sea level change, increased throw rates on dip-slip faults, or incision downstream of a stream capture event or change in land use practices. These waves radiate progressively upstream and dictate landscape response rate and form to the new boundary conditions; however, similarly shaped convexities can form in stream profiles as stationary features, fixed to sharp differences in substrate erodabilty along resistant beds or geologic contacts. In an effort to distinguish these types of convexities, we set out to contextualize large datesets of knickpoints using an algorithm that precisely defines a knickpoint, extracts these knickpoints from a landscape, and measures the dimensions of these features. The algorithm operates in accordance with the stream power incision model, specifically, utilizing a chi-plot analysis approach. This method allows for comparison between the real stream profile, and a linear best-fit line regression, representing a steady state theoretical stream functioning in accordance to uniform temporal and spatial conditions (uplift, precipitation, sediment flux, and substrate erodability). Assessing where a stream of interest is “under-steepened” and “over-steepened” with respect to a theoretical, linear, steady state profile reveals knickpoints as points of slope inflection, extractable by our algorithm.
We tested our algorithm on both a 1m LiDAR DEM, and 10m NED DEM of Santa Cruz Island (SCI), a roughly 400km2 island 45km southwest of Ventura, CA. Our algorithm identified 1745 knickpoints with an elevation drop >4m and reduced knickpoint extraction and mapping time by 99.5% in comparison with manual extraction techniques. Within resistant volcanic units, we note significant knickpoints (>40m elevation drop) fixed to resistant beds visible on the LiDAR DEM but not labeled on geologic maps. This demonstrates the complexity of interpreting the origin of knickpoint features even if geologic maps are available for a study region. Radiatively distributed knickpoints in the largest catchment on SCI show relationship to a stream capture event which generated approximately 100m of incision and rapid baselevel lowering. We show that calculating hillslope angles upstream and downstream of these radially distributed knickpoints serves as a reasonable proxy for characterizing knickpoint mobility, as migratory knickpoints display oversteepened hillslopes downstream of their location. This system shows promise to quickly identify knickpoints in a variety of geological settings; in a matter of minutes, this type of analysis will provide a regional map of potential disequilibrium landscape boundaries. These maps can provide vital preliminary information when planning for sample collection in more focused, quantitative erosion rate studies.