Magnitude and Significance of Carbon Burial in Lakes, Reservoirs, and Peatlands

Globally, lakes are currently accumulating organic carbon (OC) at an estimated annual rate of about 42 Tg·yr. Most of the OC in all but the most oligotrophic of these lakes is autochthonous, produced by primary production in the lakes. The sediments of reservoirs accumulate an additional 160 Tg annually, and peatlands contribute 96 Tg annually. These three carbon pools collectively cover less than 2% of the Earth’s surface and constitute a carbon sink of about 300 Tg·yr. Although the oceans cover 71% of the Earth’s surface, they accumulate OC at a rate of only about 100 Tg·yr. (8000 to 4000 yr B.P.). The average MARs for OC, CC, and TC (total carbon) over the past 4000 yr are 46, 36, and 82 g·m·yr, respectively. For comparison, OC MARs for the top 10 cm of eutrophic Lake Greifen, Switzerland, are 50–60 g·m·yr and were about 10 g·m·yr prior to the 1880s (Hollander et al., 1992). We calculated carbon MARs for cores from two other Minnesota lakes (Fig. 1, B and C) for which there are bulk density and carbon measurements, but much lower age resolutions. Williams Lake is hydrologically closed, and has a residence time of about 4 yr (LaBaugh et al., 1995). For the past 4000 yr, Williams Lake has been accumulating high concentrations of OC (30%–35%), and, although CaCO3 is precipitated, it is dissolved in the CO2-charged bottom waters (Schwalb et al., 1995). Initially, Williams Lake did accumulate CaCO3 (up to 75% dry weight of sediment) but that amount decreased to zero over the first half of the Holocene as the lake evolved into a closed lake and high concentrations of OC accumulated in the sediments (Schwalb et al., 1995). Nearby Shingobee Lake is hydrologically open, with a residence time of about 7 months, and has always accumulated high concentrations of CaCO3 (60%–80%) and relatively low concentrations of OC (2%–6%). The average MARs for OC, CC, and TC for Shingobee Lake over the past 4000 yr are 17, 38, and 55 g·m·yr, respectively. No CC accumulated in the sediments of Williams Lake over the past 4000 yr, but the average MARs for OC and TC are both 21 g·m·yr. Therefore, the carbon MARs in these two lakes are within the same orders of magnitude as those for Elk Lake (10–100 g·m·yr; see Fig. 1). How typical are these above-mentioned accumulation rates? The mean sediment accumulation rate in 164 midlatitude, Holocene lake sites in eastern North America (including Minnesota) reported by Webb and Webb (1988) is 81 cm·10·yr (i.e., an average of about 8 m of Holocene sediments). In contrast, profundal Holocene sediments in lakes of central Europe typically are 5–6 m thick (K. Kelts, 1997, personal commun.). The mean sediment accumulation rate for the historic period (postsettlement) in the midlatitude lake sites reported by Webb and Webb (1988) is 298 cm·10·yr (about 3 mm·yr), or about four times Holocene rates. Dry bulk densities of sediment are dependent mainly on the OC content and vary considerably. The DBDs of Holocene sediments in Elk, Williams, and Shingobee Lakes decrease rapidly with increasing OC (Fig. 2A), emphasizing the importance of bulk density measurements. However, because DBD and OC contents are inversely related, the content of OC per unit volume of sediment is relatively constant at about 20 mg·cm except for the least organic sediments (Fig. 2B). Thus, if the sedimentation rate (cm·yr) is known, the OC MAR can be estimated without measuring the OC content or the DBD. The average OC and CC concentrations in surface sediments of 46 lakes chosen as a representative sample for Minnesota are 12% and 2%, respectively (Dean et al., 1993). The relationship of bulk density to OC content (Fig. 2A) indicates that a lake sediment with 12% OC should have a DBD of about 0.2 g·cm. At an average postsettlement sedimentation rate of 3 mm·yr (the average midlatitude rate of Webb and Webb, 1988), this average sediment should have a bulksediment MAR of 600 g·m·yr, and OC and CC MARs of 72 and 12 g·m·yr, respectively. The mean OC MARs for small (<100 km2) lakes compiled by Mulholland and Elwood (1982) are 27 g·m·yr for oligotrophic lakes and 94 g·m·yr for meso-eutrophic lakes. How much carbon sequestration is occurring in Minnesota lakes alone? There are 15291 lakes in Minnesota that have an area of >10 acres (4 ha or 40 × 103 m2), and these 15 291 lakes have a total area of 3.4 × 106 acres = 1.4 × 1010 m2 (Minnesota Conservation Department, 1968). If these lakes are accumulating OC at the average Minnesota lake rate of 72 g·m·yr, the total OC accumulation is about 1012 g·yr or 1 Tg·yr. This value does not include lakes with areas <10 acres nor the extensive wetlands throughout Minnesota. Sediments in the depositional basins of the lower Great Lakes (Michigan, Huron, Erie, and Ontario) usually contain more than 2% OC of predominantly algal origin (Kemp et al., 1977; Meyers and Ishiwatari, 1993). Linear sedimentation rates and measured values of DBD, however, are more difficult to come by. Data for Lake Michigan (Rea et al., 1980; Colman et al., 1990, 1994) suggest that the youngest sediments with an average DBD of about 0.25 g·cm were deposited at an average rate of about 0.1 cm·yr for a bulk-sediment MAR of 250 g·m·yr. At 2% OC, this bulk-sediment MAR yields an OC MAR of about 5 g·m·yr (Rea et al., 1980). If we assume that the depositional basins of Lake Michigan where this OC MAR applies constitute about 75% of the area of Lake Michigan (5.8 × 104 km2), then the present OC accumulation rate in Lake Michigan is about 0.22 Tg·yr or about 22% of the rate for all Minnesota lakes. Most people would agree that the continental margins of the oceans, particularly margins under upwelling areas, are significant sinks of organic carbon. The continental margin off California under the California Current upwelling system covers an area of 4 × 1010 m2. The average Holocene OC MAR based on radiocarbondated cores with measured bulk densities from within this area was 0.06 Tg·yr (Gardner et al., 1997). In other words, the area of the California continental margin is almost three times the total area of all lakes in Minnesota larger than 10 acres, but the OC burial rate along that continental margin is only about 6% of that in Minnesota lakes. Globally, continental margins only amount to 12% of the area of the world oceans, but they are estimated to account for 44% of the present burial of OC in the oceans (Emerson and