Temporal Changes of the¹⁴C Reservoir Effect in Lakes

Abstract

Conventional radiocarbon dates for sediment samples from aquatic systems and of coeval terrestrial samples deviate from each other due to the reservoir effect. The reservoir correction is usually assumed to be constant with time for a specific aquatic system. Our studies confirm that seasonal and secular changes are frequent and are governed by the limnological conditions. Lakes have two principal sources of 14C: atmospheric CO2 and the total dissolved inorganic carbon (TDIC) of the entering groundwater and runoff. The former has values of ca. 100 pMC; the latter usually has a 14C value well below 100 pMC. Atmospheric CO2 enters the lake by exchange via its surface. The proportions of these two kinds of input determine the magnitude of the reservoir correction in freshwater lakes. It is mainly a function of the volume/surface ratio of the lake and, consequently a function of the water depth. The surface of lakes with outflow does not change when sedimentation decreases the depth of the water. The depth of Schleinsee Lake in southern Germany has decreased from 30 to 15 m since ca. 9000 bp. As a result, the reservoir correction has decreased from ca. -1550 to -580 yr. In contrast, the depth of Lake Proscansko in Croatia increased with growth of the travertine dam and the reservoir correction changed from ca. -1790 to -2650 yr during the last 8800 yr. The largest fluctuations of lake levels occur in closed lakes in arid regions when the climate changes from humid to arid and vice versa. As a result, the reservoir correction of the 14C dates for the total organic fraction from Lejía Lake in the Atacama Desert of Chile varied between <-1800 yr and -4700 yr over a period of only 1800 yr between 11,500 and 9700 bp. The corresponding reservoir correction for the marl fraction is much higher. In summary, accurate and reliable 14C dating of lake sediments requires a study of the temporal changes of the reservoir effect by analysis of both the organic and marl fractions. The most reliable 14C dates are obtained from terrestrial plant remains.

Reservoirs and Radiocarbon:14C Dating Problems in Mývatnssveit, Northern Iceland

“…In addition, it is also necessary to identify whether the FRE in Lake Myvatn has remained constant through time. Geyh et al (1998) found that FREs in lakes are not necessarily constant through time and may exhibit considerable variation due to changes in physical lake parameters (i.e. surface area and water depth).…”

The freshwater reservoir effect in radiocarbon dating

“…the ratio between lake surface and lake volume (i.e. water depth), as groundwater DIC can enter the lake from the total underground surrounding the lake, while atmospheric exchange only takes place on the surface [65]. If a river, which past FRE is to be reconstructed, runs through a lake, another complicating factor is therefore added to the temporal variability of the FRE.…”

Biogeochemical processes controlling oxygen and carbon isotopes of diatom silica in Late Glacial to Holocene lacustrine rhythmites

“…A reservoir effect of 3260 years was determined based on AMS 14 C dating of modern DIC and subsequent correction for atmospheric thermonuclear bomb tests of the late 1950s-early 1960s . However, due to changes in the volume/surface ratio of the lake that took place during the deposition of the unit 1 (Hernández et al, 2008), the reservoir effect could have changed between values of 0 to 3260 years (Geyh et al, 1998). A mid point value was calculated between the two extreme reservoir ages and this value was used for constructing the age-depth model (further details in Giralt et al, 2008 and references therein).The calibration of radiocarbon dates was performed using CALIB 5.02 software and the INTCAL98 curve.…”