Methods & Sampling:
Data were collected on four cruises conducted as day trips in 2019 (from February to August) aboard the SigmaT, as indicated by the sampling dates. The SigmaT is a local vessel with an appropriate winch for the deployment of coring equipment and Niskins. This vessel is privately owned and operated. All samples were taken at the Compass Station. Drs. Erin Black and Stephanie Kienast can be contacted about all cruises and equipment deployments.

Sediment cores ranging from 20 to 32 centimeters (cm) in length were taken with a KC Denmark four-core multi-corer at the Compass Station on February 18, April 11, May 9, and August 2 of 2019. Cores were capped, preserved, and transported back to Dalhousie University for sectioning, except for the February core, which was sectioned on the ship. The cores were extruded and sectioned in 1-4 cm intervals, with greater resolution in the upper 5 cm. In February, April, and May, one core was used for carbon and Thorium-234 (234Th) measurements. In August, two sets of paired cores (four cores total) were homogenized in 1 cm increments to provide more mass for analysis in the upper 5 cm and to test for spatial heterogeneity.

Methods follow the processing and counting procedures in Black and Buessler (2014) with details and any modifications noted here. After sectioning, sediment samples were dried in plastic sample boats at low temperatures (<50 °Celsius) over a period of 2-3 days and ground using a mortar and pestle. Samples were homogenized and then tightly packed into 50-millimeter (mm) by 9-mm falcon dishes and with masses ranging from ~8-15 grams. For the February and April sampling events, separate 234Th measurements for the 0-1 cm and 1-2 cm layers were not possible because of mass requirements for gamma counting. Therefore, there are separate carbon measurements reported for each layer, but not for 234Th activities. After a carbon subsample was taken for carbon measurements, the 0-1 cm and 1-2 cm sediment was combined and homogenized for gamma counting. The results listed for the 0-1 cm and 1-2 cm layers for the February and April sampling events represent the 234Th value for the homogenized 0-2 cm layer, not two separate values for the individual depth layers. The C:234Th ratios were determined by averaging the total carbon for the individual depth layers (0-1 cm and 1-2 cm) and dividing by the 234Th activity.

All gamma counting of sediments was completed at the Radionuclide Facility at Dartmouth College using Canberra Intrinsic High Purity Germanium Detectors. 234Th was determined at 63.3 kiloelectron volts (keV). Samples from the upper 5 cm of the cores were analyzed within 32 days. All sample activities were decay-corrected to the sampling date. A visual inspection indicated the sediments were fine-grained, organic-rich, and relatively homogenous. Porosity (i.e. water content) was determined for the upper 5 cm using the mass difference between wet and dry sediment subsamples, correcting for salt content, and assuming a sediment density of 2.65 grams per cubic centimeter (g cm-3) (Burdige 2006). Sediment uncertainties for 234Th activities are the propagation of gamma counting errors and an assessment of potential variability in porosity and density estimates.

Carbon content of sediments was analyzed in the CERC.OCEAN facility at Dalhousie University using a Costech Instruments Elemental Combustion System 4010. Approximately 4-5 mg of sediment from each depth layer were analyzed. The average carbon content (± s.d.) did not differ between acid-fumigated samples (n = 29, 5.5% ± 0.5%) and non-fumigated samples (n = 39, 5.7% ± 0.8%), suggesting that most of the total carbon is organic in nature. Therefore, it is assumed the reported 'total' carbon content here can be used interchangeably with 'organic' carbon content. For acid-fumigation tests, a drop of milli-Q water was added to each sample cup with sediment. The cups were left lid-off in a desiccator with an open bottle of concentrated hydrochloric acid overnight and then dried in a standard oven at 50 °Celsius prior to carbon analysis.