Role of Episodic Events in Long-Term Accumulation of Sediments
John A. Robbins1, D. N. Edgington2, N. R. Morehead1,
R. W. Rood3, B. J. Eadie1 and K. A. Orlandini4
1NOAA-GLERL, 2GLWI, 3CILER, 4ANL.
- Assess the role of episodic events in accumulation and focusing (winnowing) of
sediments in Lake Michigan.
- Collect sediment cores with high spatial resolution within the principal high
deposition (HIDEP) area in southeastern Lake Michigan situated about 10km offshore
from St. Joseph near or within a convergence zone (Figure 1).
- Core a 30x40 km array of HIDEP sites on several occasions before and after late
winter/early spring episodic re-suspension events (cruises in Sept. and Nov.1998, Feb.,
April and June 1999).
- Determine total inventories of long-lived fallout 137Cs (t1/2=30 years) as a measure
of long-term sediment accumulation patterns.
- Determine total inventories of short-lived atmospherically delivered 7Be (t1/2= 53
days) and water column produced 234Th (t1/2=24 days) as measures of short-term sediment
- The total inventory of 137Cs (using data from all cruises) is highly focused within
the HIDEP area (Figure 2) and looks very much like the long-term pattern of post-glacial
sediment accumulation in this area.
- Total 7Be inventories during the pre-event periods (Sept. 1998 and Feb. 1999) show
little variability within the HIDEP area and, if anything, are lower in the most focused
regions according to the 137Cs data (Figure 3).
- During the post-event periods (April and June, 1999) 7Be tends to be focused more
like 137Cs, especially by June (Figure 3). The pattern in June developed so long after
late winter that it is likely due to processes other than February/March episodic
- The June inventory of 234Th (Figure 4) was essentially unrelated to long-term
sediment focusing and tended to be far higher toward in shallower water than expected
from its production in overlying water.
J. V. Klump and J. Waples (coastal resuspension).