Debris flow magnitudes: a global catalog

The episodic occurrence of debris flow events in response to stochastic precipitation and wildfire events makes hazard prediction challenging. Previous work has shown that frequency-magnitude distributions of non-fire-related debris flows follow a power law, but less is known about the distribution of post-fire debris flows. As a first step in parameterizing hazard models, we use frequency-magnitude distributions and cumulative distribution functions to compare volumes of post-fire debris flows to non-fire-related debris flows. Due to the large number of events required to parameterize frequency-magnitude distributions, and the relatively small number of post-fire event magnitudes recorded in the literature, we collected data on 73 recent post-fire events in the field. The resulting catalog of 988 debris flow events is presented as an appendix to this article. We found that the empirical cumulative distribution function of post-fire debris flow volumes is composed of smaller events than that of non-fire-related debris flows. In addition, the slope of the frequency-magnitude distribution of post-fire debris flows is steeper than that of non-fire-related debris flows, evidence that differences in the post-fire environment tend to produce a higher proportion of small events. We propose two possible explanations: 1) post-fire events occur on shorter return intervals than debris flows in similar basins that do not experience fire, causing their distribution to shift toward smaller events due to limitations in sediment supply, or 2) fire causes changes in resisting and driving forces on a package of sediment, such that a smaller perturbation of the system is required in order for a debris flow to occur, resulting in smaller event volumes.

Isotopic survey of diagenetic carbonates, at DSDP Leg 63 Holes

Authigenic carbonates, principally calcium-rich dolomites, with extremely variable isotopic compositions were recovered in organic-rich marine sediments during Leg 63 drilling off southern California and Baja California. These carbonates occur as thin layers in fine-grained, diatomaceous sediments and siliceous rocks, mostly deposited during the Neogene. A combination of textural, geochemical, and isotopic evidence indicates these dolomites formed as cements and precipitates in shallow subsurface zones of high alkalinity spawned by abundant CO2 and methane production during progressive microbial decay of organic matter. Depths and approximate temperatures of formation estimated from oxygen isotopes are 87 to 658 meters and 10°C to 50°C, respectively. Within any sedimentary section, dolomites may form simultaneously at several depths or at different times within the same interval. Highly variable carbon isotopes (-30 to +16 per mil) reflect the isotopic reservoir in which the carbonates formed. Oxidation of organic matter through microbial reduction of sulfate at shallow depths favors light-carbon carbonates such as those at Sites 468 and 471; heavy-carbon carbonates at Site 467 most likely formed below this zone where HC**12O3**- is preferentially removed by reduction of CO2 to methane during methanogenesis. An important controlling factor is the sedimentation rate, which dictates both the preservation of organic matter on the sea floor and depth distribution of subsurface zones of organic-matter decay.