| 1) Syringe subsamples (5 mL) sealed with a butyl suba-seal were stored in an N2-flushed anaerobic jar during storage in the constant temperature room. |
| 2) These subsamples were transferred to the isotope van and kept at in situ temperature in an appropriate incubator to equilibrate. |
| 3) The control sample for each sediment depth was prechilled (4°C) prior to injection with the isotope. |
| 4) An injection microsyringe was flushed thoroughly with the isotope (at least five times), ensuring that there were no air bubbles. The subsample was placed in the injection rig, followed by the injection microsyringe, by inserting the needle through the suba-seal. An exact amount of isotope was delivered evenly along the length of the sediment in the sample by pulling back the barrel of the microsyringe completely to the stop. When removing the microsyringe from the rig, the volume injected was checked. |
| 5) Volumes of radiotracer injected into each syringe subsample and for each individual tracer was 7.2 µL (4.8 µCi) of sodium 14C-bicarbonate solution (Amersham, UK; diluted with filter-sterilized [0.2 µm], degassed distilled water) or 7.4 µL (1.5 µCi) of undiluted [1-(2)14C] acetate; Amersham UK). |
| 6) After injections, all controls were immediately frozen while all other samples were incubated in gas-tight aluminum bags containing an oxygen scrubber (Anaerocult A, Merck) for a set time (2 hr to 7 days for acetate, 0.75-35 days for bicarbonate) before activity was stopped by freezing. |
| 7) At the conclusion of the injections, the microsyringe was thoroughly rinsed with distilled water (10 times) to remove residual isotope. |
| 8) Frozen samples were transported frozen to the shore-based institute in Bristol, UK, for further processing. |