SCEC Award Number 14238 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Distinguishing Tsunami-Generated from Flood-Generated Deposits in the Laminated Sediments of Santa Barbara Basin: An Untapped Repository of Deep-Time Paleoseismic Information?
Investigator(s)
Name Organization
William Berelson University of Southern California James Dolan University of Southern California
Other Participants Nick Rollins, Tech
SCEC Priorities 1a, 2a, 2d SCEC Groups Geology, EFP
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 The Holocene sequence of finely laminated sediments in Santa Barbara Basin provides annual resolution back 300 years, decadal resolution to 2000 BP and centennial to millennial resolution back to 130,000 ybp. Embedded within Holocene dark-light laminae couplets are gray layers, 0.5-5 cm in thickness. The literature on these layers attributes them to deposition during massive floods. What if they actually represent tsunami or seismically triggered sedimentation? This would provide a remarkable time-resolved picture of tsunami/seismic frequency in the Santa Barbara region, most likely attributable to major, local earthquakes. We propose: (a) to mine and synthesize the data available regarding gray-layer ages through the Holocene and late Pleistocene; and (b) make very high resolution geochemical measurements across several gray layers to establish whether there is a geochemical and/or palynological “fingerprint” that distinguishes tsunami-derived from flood-derived deposits.
Intellectual Merit We have analyzed the geochemistry of 6 grey layers from a Piston Core (W032b10PC) collected in 2003. The exact location of these layers (in terms of dates) is not known and will be determined only after we obtain 14C ages (more on this topic below).

One question that was primary to our objectives was to examine multiple grey layers to determine if all layers were created by similar processes. Thus far, the answer appears to be ‘Yes’.
In these four panels (above) the UP direction is to the left (#0) and the x-axis is distance in cm. The boundary of each grey-layer is clearly seen in %TIC, %TOC and in the isotopic composition of the TIC. The TOC isotopic signal is much more gradual and subtle than the other geochemical measures.

All grey layers show a drop in %TIC and TOC with the onset of a grey layer and a subtle decline in %TIC during deposition of the grey layer. There is a marked increase in the light isotope of TIC during grey layer accumulation. Grey layers are not homogeneous and thus not deposited as one single, well mixed depositional event.

Mass and isotope balances show that grey layers are deposits with 4-6 times less carbonate content and 2.5 times less organic carbon content than the sediment in which they are sandwiched. Both the carbonate and organic C deposited with a grey layer is isotopically lighter than the normally accumulating sediment values.

Interpretation:

The process responsible for grey layer deposition yields a very consistent product. While it seems unlikely that floods would yield such a consistent pattern of sedimentation a tsunami-driven event seems equally unlikely to yield consistent sedimentation. If the source of the Corg in the grey layers were terrestrial, it is possible that it had a consistent δ13C signature but also unlikely. It is also unlikely that a terrestrially sourced grey layer sediment would have both TIC and TOC isotopes that were consistently lighter than the isotopic composition of ‘ambient’ sediments.

Seismic shaking and a resultant turbidity current or nephaloid layer deposit is, presently, the preferred interpretation for origin of grey layers. One premise we propose to investigate is that the grey layers are consistently older than the surrounding sediment. If this were found to be true, one conclusion is that pre-aged, fatty acid dominated sediment is preferentially mobilized, transported and deposited during these events. Mollenhauer and Eglinton (2007) demonstrate that these compounds are isotopically light compared to the ambient sediment.

Explaining the deposition of isotopically light carbonate in grey layers is perplexing. We speculate that benthic foraminiferal carbonate (very small fragments) may be the source of the light TIC isotope signal.
Broader Impacts Work involves one technician and one (female) undergraduate. The undergrad is preparing posters for presentation.
Exemplary Figure Isotope figure