Climate-related cyclic deposition of carbonate and organic matter in Holocene lacustrine sediment, Lower Michigan, USA
Jennifer A. Nelson, Kathy Licht, Catherine Yansa, and Gabriel Fillippelli
Records from lake sediment cores are critical for assessing the
relative stability of climate and ecosystems over the Holocene. Duck Lake in
south-central Lower Michigan, USA, was the focus of a study that identified how
changes in the geochemical variables in lake sediments relate to variations in
regional climate and local land use during the Holocene. More than 8.5 m of
lacustrine sediment were recovered using Livingston and freeze corers and
analyzed for organic carbon, inorganic (carbonate) carbon, total nitrogen, and
trace metals. Repeating packages of sediment (110 cm thick) that grade from
light (inorganic carbon-rich) to dark (organic carbon-rich) were found from the
surface to a depth of about 8 m. Variations in the highresolution gray scale
data from core X-radiographs are highly correlated to the relative amount of
inorganic carbon. Geochemical analyses of the upper 8.5 m of sediment revealed a
wide range of values: 0.0510.6% for inorganic carbon (i.e. 0.589% calcium
carbonate) and 1.128% for organic carbon (i.e. 2.770% organic matter). Organic
carbon to nitrogen ratios indicate that most of the sediment organic matter is
produced within the lake. A core chronology based on eight AMS radiocarbon dates
shows low sediment accumulation rates (0.05 cm/ year) from 10,000 to 3,800 cal
year BP and higher sediment accumulation rates (0.10.3 cm/year) from 3,800 cal
year BP to present. We suggest that carbonate accumulates during relatively dry
times, whereas organic matter accumulation dominates when nutrient input to the
lake is enhanced by wetter climate. The Duck Lake core records a distinct low
point in inorganic carbon deposition that may be related to the 8.2 ka cooling
event now documented from several sites in North America. Spectral analysis of
gray scale values shows significant *200-year periodicities over the past 8,000
years, hypothesized to result from climate changes induced by solar forcing.
Concentrations of trace metals (e.g. lead, iron, copper, zinc) indicate the
onset of regional anthropogenic influence about 150 cal year BP.