Megnetic susceptibility of a subtropical South Atlantic transect

The Mid-Pleistocene transition (MPT) of the global climate system, initiated by a shift towards much larger northern hemisphere ice shields at around 920 ka and ending with predominance of 100 kyr ice age cyclicity since about 640 ka, is one of the fundamental enigmas in Quaternary climate evolution. Climate proxy records not exclusively linked to global ice volume are necessary to advance understanding of the MPT. Here we present a high-resolution Pleistocene magnetic susceptibility time series of 12 sediment cores from the subtropical South Atlantic essentially reflecting dissolution driven variations in carbonate accumulation controlled by changes in deep water circulation. In addition to characteristics known from delta18O records, the data sets reveal three remarkable features intimately related to the MPT: (1) an all-Pleistocene minimum of carbonate accumulation in the South Atlantic at 920 ka, (2) a MPT interim state of reduced carbonate deposition, indicating that the MPT period may have been a discrete state of the Pleistocene deep water circulation and climate system and (3) a terminal MPT event at around 540-530 ka documented in several peculiarities such as thick laminated layers of the giant diatom Ethmodiscus rex.

(Table 1) Age, fork length, tissue d15N and d13C and mercury content of arctic char (Salvelinus alpinus) across the Canadian Arctic

Among-lake variation in mercury (Hg) concentrations in landlocked Arctic char was examined in 27 char populations from remote lakes across the Canadian Arctic. A total of 520 landlocked Arctic char were collected from 27 lakes, as well as sediments and surface water from a subset of lakes in 1999, 2002, and 2005 to 2007. Size, length, age, and trophic position (d15N) of individual char were determined and relationships with total Hg (THg) concentrations investigated, to identify a common covariate for adjustment using analysis of covariance (ANCOVA). A subset of 216 char from 24 populations was used for spatial comparison, after length-adjustment. The influence of trophic position and food web length and abiotic characteristics such as location, geomorphology, lake area, catchment area, catchment-to-lake area ratio of the lakes on adjusted THg concentrations in char muscle tissue were then evaluated. Arctic char from Amituk Lake (Cornwallis Island) had the highest Hg concentrations (1.31 µg/g wet wt), while Tessisoak Lake (Labrador, 0.07 µg/g wet wt) had the lowest. Concentrations of THg were positively correlated with size, d15N, and age, respectively, in 88,71, and 58% of 24 char populations. Length and d15N were correlated in 67% of 24 char populations. Food chain length did not explain the differences in length-adjusted THg concentrations in char. No relationships between adjusted THg concentrations in char and latitude or longitude were found, however, THg concentrations in char showed a positive correlation with catchment-to-lake area ratio. Furthermore, we conclude that inputs from the surrounding environment may influence THg concentrations, and will ultimately affect THg concentrations in char as a result of predicted climate-driven changes that may occur in Arctic lake watersheds.