What stratal thickness tells us about

sedimentary processes in the vent complex at

MC118Charlotte 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 Y1-Y2 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 layer”at 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

SESW

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

9.5-14 Ka2.3-9.5 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

0408-01

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

Nor

mal

Clam shell