a, 14C and 210Pb dating. The 14C ages (with 1σ ranges; grey, rejected dates) from planktic foraminifera yield a modern core-top age and indicate an average sedimentation rate over the past 1,000 years of 320 cm kyr−1 (dashed line). The presence throughout the core of abundant lithogenic grains in the >150-μm fraction—along with the coarse sortable-silt mean grain size values—suggests that some reworking of foraminifera has probably occurred, resulting in average 14C ages that may be slightly (around 50 years) older than their final depositional age, consistent with the fact that the 210Pb dates do not splice smoothly into the 14C ages (the 14C ages appear slightly too old). The final age model was therefore based on the 210Pb ages for the past century, and was then simply extrapolated back in time using the linear sedimentation rate of 320 cm kyr−1. Given that none of our findings depend on close age control in the older section of this core (that is, before ad 1880), this uncertainty (with converted 14C ages being about 50 years older than the extrapolated linear age model) does not affect our conclusions. b, Left, the age model for the top 80 cm of core 56JPC is based on 210Pb dating of bulk sediment, using the constant initial concentration (CIC) method (rejecting the date at 47 cm, which probably indicates a burrow). A simple two-segment linear fit to the 210Pb dates is adopted (rather than point-to-point interpolation or a spline) because sedimentological evidence—an abrupt increase in the percentage of coarse fraction at 23 cm depth, not observed elsewhere in the core—is indicative of a step change in the sedimentation rate. Horizontal dashed lines denote the depths of the segments at which the sedimentation rate is inferred to change. Centre, further support for the age model of 56JPC over the past century comes from the down-core abundance profile of spheroidal carbonaceous particles (SCPs, derived from high-temperature fossil fuel combustion, counted as described39), which ramped up from the mid to late 1800s and peaked in the 1950s to 1970s (40 cm to 25 cm) before declining over recent decades, consistent with the 210Pb-based age model. Right, the occurrence of 137Cs in the top 40 cm or so of the core is also consistent with the 210Pb-based age of around 1950 at 40 cm. The age uncertainty (1σ) for the past 60 years of the core is estimated at ±2–3 years. We note that the sediment core top is at 3 cm depth in the core-liner.