High‐latitude forcing of interior ocean δ 13 C

Transit time distribution probability density functions (TTDs) are used to investigate the possible role of changing boundary conditions in driving the δ 13 C signal in the interior of a steady state ocean. We use idealized examples to investigate the general question of how a conservative tracer propagates from the surface ocean to interior ocean and to illustrate how a given tracer boundary signal will be “filtered” with increasing distance from its source region. We show that tracers in the deep southeast Atlantic Ocean will respond much more strongly to changes in the surface Southern Ocean than to changes in the high‐latitude North Atlantic, while the opposite is true for waters at intermediate depths. The impact of a change in the Southern Ocean surface δ 13 C on a profile from the western South Atlantic is estimated using model‐derived transit time distributions, and it is shown that significant deep ocean δ 13 C variations can be expected on glacial‐interglacial timescales, even under a steady state circulation regime. Records of δ 13 C from the high‐latitude North Atlantic and Southern Ocean are used as a proxy for glacial‐interglacial changes in the surface ocean boundary condition in regions of deepwater formation. By convolving these high‐latitude boundary conditions with model‐derived TTDs, we are able to explain a significant part of the observed variability in benthic δ 13 C records spanning the last glacial cycle(s) from locations as diverse as the equatorial Atlantic Ocean, the Cape Basin, and the equatorial Pacific. This suggests that changing boundary conditions may be driving a significant fraction of benthic δ 13 C variability previously attributed to changes in ocean circulation. Furthermore, we show that our results predict a slightly higher δ 13 C than observed in high‐productivity regions, consistent with the concept of a productivity‐induced low‐δ 13 C overprint.