Holocene Climate History of North Central Minnesota, Interpreted From δ D and δ \18O of Lake Sediments
Abstract
Stable isotopes of authigenic carbonates in lake sediments are routinely used to reconstruct past climate. When carbonates are absent, other stable isotope records such as cellulose oxygen and δ D of specific organic compounds such as palmitic acid (PA) are being used to infer water temperature. Here we show that δ D of PA can be used to reconstruct aridity in appropriate climatic settings. Cored sediment from Moody Lake, located near the base of the Crow Wing River watershed, was analyzed for lithological variations indicative of lake-level, total organic and inorganic carbon, pollen, O and C isotopes of authigenic calcite and H isotopes of PA. Modern sediments in Moody Lake contain no authigenic carbonates, but lower sections contain 5 ∼10% total inorganic carbon. Total organic carbon varies between 10 ∼35% in the deep-water core. The δ \18O values varies from -8‰ at about 11ka \14C increasing to -2‰ at ca. 4ka \14C. The δ D values are approximately -210‰ at 11ka \14C and -180‰ at the top, with a peak of -170‰ in the mid-section. The δ D values of PA can be converted to that of water using a published equation. If we assume that authigenic carbonate is formed only during warm summer months, say at 25° C, the small temperature dependence of carbonate oxygen isotope fractionation allows us to estimate the δ \18O of water without introducing a large error, given the range of 6‰ in δ \18O values. When the two calculated isotope values are compared to the Global Meteoric Water Line and to local (Minneapolis) precipitation, they fall on an evaporation line of slope ∼5, a typical slope observed for the region. Mid-Holocene values are farthest from the local meteoric precipitation indicating greater aridity, and early Holocene values closer to the meteoric precipitation. The overall pattern is interpreted as reflecting cooler and wetter conditions in the early Holocene, gradually increasing temperatures toward the mid-Holocene, much higher aridity during the mid-Holocene, and a return to cooler and wetter conditions during the late-Holocene. Lake-level history derived from stratigraphic correlation of sand layers in shoreline transects is broadly coincident with mid-Holocene aridity. The start of the increase of oak pollen and the highest concentration of herb pollen indicating increased aridity and warmth at ∼8ka \14C coincide with the beginning of the highest values in δ D and δ \18O.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMPP43A0599Z
- Keywords:
-
- 3344 Paleoclimatology;
- 1833 Hydroclimatology;
- 1040 Isotopic composition/chemistry