Table 2. Radiocarbon signatures (Δ¹⁴C values) and proportions of RDOC in mPMA OM.

Proportions assume mPMA OM result from mixtures of OM recently produced in the near-surface ocean (Δ¹⁴C of DIC at 5 m) and RDOC, with RDOC Δ¹⁴C values constrained between those of fossil molecules (−1000‰) and DOC collected from 2500 m (averaging −457 ± 8‰, n = 3; table S6). Near-surface DOC Δ¹⁴C values are shown for comparison. All mPMA Δ¹⁴C values were measured from individual samples generated over the course of 24 hours, except where noted, and reported with single SDs propagated from blank-correction calculations. All DIC and DOC Δ¹⁴C values are reported as the average and single SD of duplicate measurements at each station, except where noted. All Δ¹⁴C identification numbers are reported in tables S1 and S4 to S7.

Station	Δ14C (‰)			Proportion of RDOC in mPMA
	DIC	DOC	mPMA	Fossil RDOC	2500-m DOC
GB	19 ± 2	−248 ± 14	−140 ± 23*	0.16 ± 0.02	0.33 ± 0.05
	19 ± 2	−248 ± 14	−177 ± 56†	0.19 ± 0.06	0.41 ± 0.12
	19 ± 2	−248 ± 14	−188 ± 47	0.20 ± 0.05	0.43 ± 0.10
SSW	43 ± 2‡	−291 ± 4§	−166 ± 20	0.20 ± 0.02	0.42 ± 0.04
SSN	43 ± 2‡	−234 ± 7||	−149 ± 69	0.18 ± 0.07	0.38 ± 0.14
BI	23 ± 3	−227 ± 4	−50 ± 11	0.07 ± 0.01	0.15 ± 0.02
	23 ± 3	−227 ± 4	−6 ± 29	0.03 ± 0.03	0.06 ± 0.06
Averages excluding coastal Rhode Island			−164 ± 20	0.19 ± 0.02	0.40 ± 0.04

*mPMA generated during daytime (~12 hours between sunrise and sunset).

†mPMA generated during nighttime (~12 hours between sunset and sunrise).

‡Individual measurements at SSW and SSN were not significantly different and therefore averaged (n = 2).

§Individual measurement with uncertainty reported as single SD propagated from blank-correction calculations.

||Average and SD of triplicate measurements (n = 3).