John A. Robbins¹, D. N. Edgington², N. R. Morehead¹, R. W. Rood³, B. J. Eadie¹ and K. A. Orlandini^4
1NOAA-GLERL, ²GLWI, ³CILER, ⁴ANL.

Objectives:

• Assess the role of episodic events in accumulation and focusing (winnowing) of sediments in Lake Michigan.

Approach:

• 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 (t_1/2=30 years) as a measure of long-term sediment accumulation patterns.
• Determine total inventories of short-lived atmospherically delivered ⁷Be (t_1/2= 53 days) and water column produced ²³⁴Th (t_1/2=24 days) as measures of short-term sediment deposition patterns.

Results:

• 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 re-suspension events.
• 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.