What stratal thickness tells us about sedimentary processes in the vent complex at MC118 Charlotte Brunner (USM) Wes Ingram (Devon Energy) Stephen Meyer (UW-Madison)

Goals and objectives from the April 30, 2009 meeting Goal: Test hypotheses of fluid flow and hydrate formation along faults (i.e., Jim Knapp et al., IODP planning letter, 2009). One of several objectives: Core for evidence of present-day and latest Quaternary biogeophysical activity in the vent complex (due to venting, faulting, and related processes). What processes are active? When did activity start/end at the several vents? – Isopach maps.

Data Cores taken by the Consortium in and around the vent complex. Cores taken by Wes Ingram and Stephen Meyer (see isopach maps of region surrounding the vent complex; Marine Geology, in press, corrected proof available online). Sediment composition and correlative horizons in the regional stratigraphy (work of the past 4 years).

Gravity cores 13 cores outside the vent complex. ~40 cores inside the vent complex.

Tools: indicator of vent activity Isopach maps of sediment thickness between isochronous horizons: thick/thin units imply – Fast/slow rates of net sediment accumulation Sediment suspended and swept away during fluid/gas expulsion, Sediment re-suspended and transported downslope during faulting (earthquakes); – Differential compaction due to physical (collapse of pore spaces) or biochemical processes including transformation of solids to gas or fluid phases.

The red layer Distinguished by – Color; – Texture (clay-rich); – Higher density; – Reworked pre-Quaternary nannofossils (Marchitto and Wei, 1995); – Consistent occurrence immediately above the Y 1 -Y 2 planktonic foraminiferal biostratigraphic boundary (Brunner, 2007). Emplaced regionally by hyperpycnal flows at ~14 Ka during the de-glaciation (Aharon, 2006).

Indicator of activity Isopach map: depth to the red layerat 14 Ka. Thinnest sections (>120 cm) and slowest net sedimentation rates at the SE and SW vents (implies both vents active after 14 Ka). Thinness surprising! Why? Contours: 20- cm intervals SE SW NW

Accommodation space The sedimentation rate is slower in the vent complex, despite its greater accommodation space. Implies that processes in addition to faulting must be at work.

Interpretations Physico-chemical processes have caused shortening of the sediment column in the vent complex compared to the surrounding area: – Expulsion of gas/fluids may re-suspend sediment, which drifts from the area in suspension or as dilute turbidity flows on the seafloor; – Fault motion and earthquake shaking can cause compaction, re-suspension, and mass wasting; – Growth and dissolution of methane hydrate deposits can cause re-suspension and mass wasting. – Some sediment thinning may be due to chemical transformation of solids into solutes and gases through diagenesis or into more dense solids (hardgrounds).

Future work Examine high-resolution bathymetry and side- scan for evidence (and pathways) of mass wasting. Take additional cores in data gaps surrounding the vent complex and downslope to delimit the region of slow sedimentation and to find possible depo-centers of re-suspended material. Extend additional isochrons into the vent complex to determine the timing of vent activity. With guidance from physical properties folks, estimate volume/mass changes possible due to diagenetic transformations.

Shifts in focus of regional sed. minimum through time Ka Ka0-2.3 Ka Ingram, Meyers, Brunner, and Martens, in press, Marine Geology. The corrected proof is online. 16–30 cm/Ka6–14 cm/Ka2–11 cm/Ka

Faults White: Nodules and diagenesis at surface. Pink: diagenesis in subsurface but not at surface. Yellow: no core recovery or very short (<5 cm). Indicators of biochemical activity SE SW NW (Macelloni et al., in preparation)

Tools: indicators of vent activity Evidence of elevated sediment temperature associated with fluid flow: expressed by diagenesis of clay- and silt-size material. Evidence of anaerobic oxidation of methane to carbonate: expressed by nodules and hardgrounds. Indicated by – No core recovery or unusually short cores; – Carbonate nodules in cores. Evidence of fossil seep fauna, especially chemosynthetic vescomyid clam shells. Distribution correlates well with modern geomorphic evidence of venting and faulting (Macelloni et al., in preparation) and could trace Pleistocene history of venting. Nodule Altered color Normal Clam shell