Sediments (spanning a minimum of 13.3 ka) from the St. Anna Trough (Northern Kara Sea). The site has experienced extensive fluctuation in the delivery of river-derived organic matter (OM), caused by a eustatic change in sea level. This is in line with the record of the concentration of the isoprenoid GDGT, crenarchaeol, produced by marine Thaumarchaota, which was low at the bottom of the core, increasing gradually in the most shallow unit. The concentration of branched (br)GDGTs showed an opposite trend and a marked shift in distribution. The deepest sediments (>10 ka), with a distribution currently encountered in surface sediments in front of the Yenisei River, are characterized by terrigenous brGDGTs, whereas the distribution in the shallowest unit (<10 ka) is strongly influenced by marine, in-situ brGDGTs. During the shift from terrigenous to marine-sourced brGDGTs, there was one horizon where a pronounced shift in the brGDGT distribution was observed and the brGDGT concentration significantly decreased. As the brGDGTs delivered to the current Kara Sea system are derived from several sources, we postulate that a temporary change in the relative importance of the brGDGT sources happened during this interval. Both in-situ production and changing brGDGT provenance have implications for paleoclimate reconstruction using brGDGTs. In-situ production of marine brGDGTs results in a higher reconstructed pH. However, these in-situ produced brGDGTs did not influence the reconstructed mean annual air temperature (MAT), when the MATmrs calibration was used. Changes in the relative contribution of brGDGT sub-pools were shown, however, to influence both soil pH reconstruction and MAT reconstruction.