Paleoclimate changes inferred from stable isotopes and magnetic properties of organic-rich lake sediments in Arctic Norway

Abstract

Stable isotope measures in organic matter are frequently used as indicators of past climate change. Although such analyses can provide valuable information, there is considerable uncertainty associated with studies of organic-rich sediments, especially those from Arctic lakes and bogs. We studied stable isotopes of carbon and nitrogen, and magnetic properties in a sediment core from a small alkaline lake with a high sedimentation rate, Lake Nattmåls-vatn, Norway. There is good correspondence among the different sediment variables during the late glacial, and they seemingly reflect major climate variations such as the Allerød Interstade and the Younger Dryas, as well as the transition into the current interglacial. During the early Holocene, however, these relationships are more complex and d 13 C and d 15 N values do not stabilize until *7,500 cal year BP. A significant excursion in all variables occurs between 6,850 and 6,500 cal year BP and is interpreted to represent climate deterioration. Holocene d13 C values vary little and indicate that isotopically-depleted dissolved inorganic carbon (DIC) in the lake, possibly influenced by methanotrophy and high pCO 2 , dominated the lake's carbon cycle. Holocene d 15 N is similarly muted, likely due to the availability of abundant dissolved nitrogen. Bulk organic matter is probably dominated by phytoplankton remains produced beneath the ice cover in late spring and during ice breakup when isotopicallydepleted DIC, pCO 2 and ammonium availability were maximal. Thus, use of d 13 C and d 15 N as indicators of Holocene paleoclimate and paleoproductivity variation can be challenging in a lake such as Nattmåls-vatn, where ice cover isolates the basin for large parts of the year, allowing dissolved respiratory gases to accumulate in the water column. In contrast, magnetic variables appear to better track climate variations. In particular, runoff-driven influx of minerogenic sediments shows high variability that can be attributed to regional changes in Holocene winter precipitation. The most striking shifts occur between 4,000 and 2,300 cal year BP. M. R. Talbot Deceased.Electronic supplementary material The online version of this article

Evaluating Holocene climate change in northern Norway using sediment records from two contrasting lake systems

“…Fiskebølvatnet shows the greatest input from runoff beginning at 4.3 ka and lasting until 1.7 ka. This interval closely corresponds with the period of increased winter precipitation, from 3.8 to 2.0 ka, interpreted from reconstructed glacier equilibriumline altitudes from the Lyngen Peninsula and an increase in winter precipitation from 4.0 to 2.3 ka interpreted from enhance minerogenic influx into another lacustrine system in northern Norway (Janbu et al 2011).…”

High‐Arctic climate conditions for the last 7000 years inferred from multi‐proxy analysis of the Bliss Lake record, North Greenland

“…Wolfe et al , 1999). This implies that the dissolved organic carbon (DIC) pool is not limiting for OM productivity, suggesting that winter ice cover may have contributed to the DIC availability through carbon respiration, leading to accumulation of dissolved CO 2 under ice (Janbu et al , 2011). Accumulation of dissolved CO 2 under ice may result in concentration well in excess of those resulting from equilibrium with the atmosphere (Striegl et al , 2001; Wetzel, 2001; Huttunen et al , 2003).…”

“…Accumulation of dissolved CO 2 under ice may result in concentration well in excess of those resulting from equilibrium with the atmosphere (Striegl et al , 2001; Wetzel, 2001; Huttunen et al , 2003). It is likely that low δ 13 C values are assimilated from DIC under ice and subsequently ice‐out where dissolved nutrients were at their highest levels (Janbu et al , 2011). Hence low δ 13 C values in zone 3 are likely reflecting periods with prolonged ice cover, in contrast to periods with enriched δ 13 C reflecting periods with reduced winter ice cover from 7220 cal.…”

Paleolimnological record as an indication of incipient eutrophication in an oligotrophic subtropical coastal lake in Southern Brazil

“…In Lake Nattmålsvatn (Norway) sediments, C:N ratios ranged from 8 to 13, which was also considered an indication that OM comes predominantly from aquatic sources and that in-lake production was high (Janbu et al 2011). The C:N ratios in Lake Ledvica (Slovenia) decreased in contemporary sediment, and this was associated to either a lower terrestrial contribution or an increased primary production in more recent sediments (Vreca and Muri 2006).…”