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