Deglacial floods in the Beaufort Sea preceded Younger Dryas cooling

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Nature Geosciencevolume 11 (2018): 599-604, doi:10.1038/s41561-018-0169-6.The Younger Dryas cooling at ~13 ka, after 2 kyr of postglacial warming, is a century-old climate problem. The Younger Dryas is thought to have resulted from a slow-down of the Atlantic meridional overturning circulation in response to a sudden flood of Laurentide Ice Sheet meltwater that reached the Nordic Seas. Although there is no oxygen isotope evidence in planktonic foraminifera from the open western North Atlantic for a local source of meltwater from the Gulf of St. Lawrence where it was predicted, we report here that the eastern Beaufort Sea contains the long-sought signal of 18O-depleted water. Beginning at ~12.94 ± 0.15 ka, oxygen isotopes in planktonic foraminifera from two sediment cores as well as sediment and seismic data indicate a flood of melt water, ice and sediment to the Arctic via Mackenzie River that lasted about 700 years. The minimum in oxygen isotope ratios lasted ~130 years. The floodwater would have travelled north along the Canadian Archipelago, and through Fram Strait to the Nordic Seas where freshening and freezing near sites of deepwater formation would have suppressed convection, and caused the Younger Dryas cooling by reducing the meridional overturningThis research was funded by NSF grants ARC 1204045 to L.D.K., and ARC 1203944 to N.W.D

Recency of Faulting and Subsurface Architecture of the San Diego Bay Pull-Apart Basin, California, USA

In Southern California, plate boundary motion between the North American and Pacific plates is distributed across several sub-parallel fault systems. The offshore faults of the California Continental Borderland (CCB) are thought to accommodate ∼10–15% of the total plate boundary motion, but the exact distribution of slip and the mechanics of slip partitioning remain uncertain. The Newport-Inglewood-Rose Canyon fault is the easternmost fault within the CCB whose southern segment splays out into a complex network of faults beneath San Diego Bay. A pull-apart basin model between the Rose Canyon and the offshore Descanso fault has been used to explain prominent fault orientations and subsidence beneath San Diego Bay; however, this model does not account for faults in the southern portion of the bay or faulting east of the bay. To investigate the characteristics of faulting and stratigraphic architecture beneath San Diego Bay, we combined a suite of reprocessed legacy airgun multi-channel seismic profiles and high-resolution Chirp data, with age and lithology controls from geotechnical boreholes and shallow sub-surface vibracores. This combined dataset is used to create gridded horizon surfaces, fault maps, and perform a kinematic fault analysis. The structure beneath San Diego Bay is dominated by down-to-the-east motion on normal faults that can be separated into two distinct groups. The strikes of these two fault groups can be explained with a double pull-apart basin model for San Diego Bay. In our conceptual model, the western portion of San Diego Bay is controlled by a right-step between the Rose Canyon and Descanso faults, which matches both observations and predictions from laboratory models. The eastern portion of San Diego Bay appears to be controlled by an inferred step-over between the Rose Canyon and San Miguel-Vallecitos faults and displays distinct fault strike orientations, which kinematic analysis indicates should have a significant component of strike-slip partitioning that is not detectable in the seismic data. The potential of a Rose Canyon-San Miguel-Vallecitos fault connection would effectively cut the stepover distance in half and have important implications for the seismic hazard of the San Diego-Tijuana metropolitan area (population ∼3 million people)

Geophysical and geological studies of margin evolution: impacts of sea level fluctuations, tectonic deformation, and climate

Understanding modern process that shape continental margins has both scientific and societal relevance. By studying modern marine deposits, where the stratigraphy can be imaged at unprecedented scales and accurately dated with radiocarbon techniques, we can assess the link between the forcing factors and the consequent deposit. Toward this goal, this thesis presents geological and geophysical data that provide new insights into how different forcing mechanisms (e.g., sea level fluctuations, tectonic deformation, climate variability, glacial lake drainage) shaped continental margins. Three different margins were examined: San Onofre, CA, Block Island Sound, RI, and the Beaufort Margin, Arctic Ocean. In San Onofre, CHIRP data on the shelf imaged multiple transgressive deposits that gave insights into their controlling processes. Although numerous faults dissect the area, there is no evidence of recent activity or deformation on the fault systems offshore San Onofre. Instead, the rate of sea level rise, sediment supply, and preexisting morphology were found to be the controlling factors in sediment dispersal. Compressional features were imaged along the Cristianitos fault that runs from onshore to offshore in the study area. This suggests that the fault is actually a strike-slip fault with a down-to-the-northwest dip-slip component, versus a simple normal fault as purported.In Block Island Sound, CHIRP data, scanned 3.5 kHz seismic profiles, and bathymetry data provide important insights into the morphologic evolution of the sound and the draining manner of glacial lakes that formerly occupied Block Island Sound and neighboring Long Island Sound. Architecture of sediment units imaged in the seismic data and erosive features imaged by bathymetry data suggest a rapid draining of the glacial lakes. Only partial infill of erosive features in Block Island Sound suggests a rapid transgression and/or a lack of modern sediment deposition.Along the Beaufort Margin, seismic data and sediment cores were used to constrain the deglacial history of the area. Oxygen isotopes document a large input of freshwater that entered the Arctic via the Mackenzie River. Timing of this event correlates with the onset of the Younger Dryas cold period, suggesting that this flood may have triggered the attendant climate cool period. This study reveals that the slope west of the Mackenzie River has higher rates of Holocene sedimentation, suggesting it is influenced more by Barrow Canyon and continental runoff. The slope east of the Mackenzie River has a much stronger record of Mackenzie input, including a rapid depositional event. Ice rafted debris from the Amundsen Gulf is observed throughout most of the margin, but more prevalent in the eastern Beaufort