Carbon measurements

Samples were collected from the field following the stratigraphy of Guex et al.25,26 (see Supplementary Fig. 4 for an image of the collection site). Samples were inspected and those with veins and weathered surfaces were removed. Samples were crushed in a jaw crusher and then pulverized in an agate ball mill at the University of Southern California. An aliquot of powder from each sample (∼0.5 g) was dissolved in 40 ml of 1 M hydrochloric acid and heated to 70 °C for 4 h to remove all carbonate mineral phases. This method is similar to that described by Ward et al.27. Samples were washed with deionized water three times and dried at 50 °C.

Weight percent organic carbon was determined on decarbonated powder using a Picarro cavity ring down spectrometer (G2131-i) coupled via a Picarro Liason (A0301) to a Costech Elemental Combustion System (EA 4010). This determination of organic carbon content was converted to a value of % TOC taking into account the amount of carbonate loss during acid treatment. Errors were calculated by replicate analyses of samples and standards. The 1 s.d. uncertainty was assigned as 10% of the reported value, which takes into account uncertainties associated with decarbonation. Standards included both internal CaCO 3 standards and the USGS-40 reference material (L-glutamic acid).

The isotopic composition of organic carbon was also determined using the Picarro cavity ring down spectrometer and is reported in delta notation (δ13C org ) relative to the Vienna Pee Dee Belemnite standard. The uncertainty on the δ13C org values was assessed from replicate runs of standards (including NBS-18 calcite, USGS-40 and internal carbonate standards) and samples. Replicate analyses were run on 33% of the samples. Standard deviation on replicate analyses was on average <0.1‰. Uncertainties and blanks associated with this methodology are further discussed in Subhas et al.47

Mercury concentration measurements

Samples were inspected, crushed and pulverized at the University of Southern California, as described above. Total Hg was measured using a Hydra II c Direct Mercury Analyzer (Teledyne Leeman Labs) at the University of Toronto. Within the Hydra II c , samples were combusted in two stages under an oxygen flow of 350 ml min−1. First they were heated to 300 °C for 30–60 s, and then decomposed at 800 °C for 300–500 s. After combustion, the evolved gases were carried through a heated catalyst tube to remove possible interferences (for example, halogen compounds, sulfur oxides, nitrous oxides) and Hg was captured on a gold amalgamation trap while combustion gases were removed from the detection cell. The gold trap was then heated for 30 s at 600 °C to release Hg. Hg was carried to the detection cell where absorbance from a mercury lamp was measured at 253.7 nm.

Calibration was performed using a fresh, gravimetrically prepared NIST 3133 Hg standard in a 0.25% L-cysteine solution. Blank absorbance was <2% of typical sample signals and always <4%. Sample boats were periodically re-combusted to check that all available Hg had been released during the initial analysis. To determine measurement precision, the NIST 3133 L-cysteine solution was periodically combusted and analysed alongside samples. The measured concentrations of the NIST 3133 standard are within 5% of nominal values.

Samples measured more than once are reported as the mean of duplicate measurements (Supplementary Table 1). Reproducibility of sample concentrations was better than 10%. To check measurement accuracy, powders of NIST SRM 1944 (New York/New Jersey Waterway Sediment) and NIST SRM 1646a (Estuarine Sediment) were repeatedly combusted over the period of sample analysis. The average value for NIST 1944 was 3,496±334 p.p.b. (2 s.d., n=2), which is within the certified value of 3,400±500 p.p.b., and the average value for NIST 1646a was 27.7±2.8 p.p.b. (2 s.d., n=9). Although NIST 1646a is not certified for Hg, we used it as in-house external standard because our batch had a similar Hg content to the samples. The measured concentrations of NIST 1646a are consistent with the long-term values obtained on this standard in our laboratory. Based on the reproducibility of samples and external standards, errors on Hg concentration measurements are estimated to be 10% (2 s.d.).

Mercury isotope nomenclature

Mercury isotope compositions are reported using nomenclature suggested by Blum and Bergquist48. Isotopic compositions are reported using δ-notation relative to the NIST SRM 3133 standard according to equation (1):

where x is the mass number of each Hg isotope from 199Hg to 204Hg. We use δ202Hg to report MDF. MIF is reported as ΔxHg, which is defined using equation (2):

where x is the mass number of each Hg isotope (199, 200, 201 and 204) and β is the scaling constant used to estimate theoretical MDF based on kinetic mass fractionation49. β is 0.2520, 0.5024, 0.7520 and 1.493 for 199Hg, 200Hg, 201Hg and 204Hg, respectively.

Mercury isotope measurements

Before isotope analysis, Hg was extracted and purified from samples by combustion separation using the furnace module of the Hydra II c with the gold trap removed. The decomposition procedure was the same as described for the Hg concentration measurements. To trap Hg, the gas outflow containing elemental Hg was sparged directly into a freshly prepared solution of ∼10% trace metal grade H 2 SO 4 (v/v) and ∼1% KMnO 4 (w/w), where the Hg0 gas was oxidized to Hg(II). After the combustion of each sample, 50 μl of Milli-Q water was loaded into a nickel boat and combusted according to the same procedures as samples to ensure removal of any residual Hg in the furnace. During this step, the line linking the gas outflow to the sparger was also heated with a heat gun to ensure full recovery of Hg.

Aqueous solutions of NIST 3133, powders of NIST 1646a and blanks were combusted and trapped alongside samples as procedural standards and blanks. Procedural blanks were <0.02 ng g−1, which is <1–2% of the sample Hg. Recovery of Hg from samples and process standards was checked by neutralizing an aliquot of each solution with NH 2 OH-HCl immediately after trapping and measuring its concentration using a Tekran 2600 cold vapour atomic fluorescence spectrometer. The recoveries of samples were 99.3±10.6%, (2 s.d., n=35) and of procedural standards were 99.6±4.8% (2 s.d., n=8). The ∼10% variation in sample recoveries reflects both the uncertainty in concentration method and sample heterogeneity.

Isotopic analysis was conducted using a cold vapour multi-collector inductively coupled plasma mass spectrometer (Neptune Plus, Thermo-Finnigan) at the University of Toronto. Sample solutions were first neutralized with NH 2 OH-HCl in order to reduce KMnO 4 and then diluted to 1–2 ng g−1 using a pre-neutralized 1% KMnO 4 solution (the same matrix as samples). Hg was introduced into the plasma as Hg(0) using online SnCl 2 reduction and Hg(0) vapour separation. To correct for instrumental mass bias, we used an internal Tl standard (NIST 997; introduced as a desolvated aerosol) and strict standard-sample bracketing with the NIST 3133 Hg standard. In addition, an in-house secondary aqueous Hg standard (J.T.Baker Chemicals) was measured at least seven times in each analytical session to determine the external reproducibility of the method. Both the NIST 3133 bracketing standards and the J.T.Baker Hg standards were prepared in the same matrix solution as samples and procedural standards. Signal concentrations and intensities of all standards and samples were matched within 10%. Isobaric interference from 204Pb was monitored using 206Pb, but was always negligible (correction never altered the calculated δ204Hg). On-peak blank corrections were made on all Hg and Pb masses and the Hg intensities of the blank measurements were monitored to ensure negligible carry-over and build up of Hg.

The average value of the JT Baker Hg standard over all analytical sessions was −0.60±0.09‰ for δ202Hg and 0.02±0.03‰ for Δ199Hg (2 s.d., n=31; Supplementary Table 2), which is consistent with previous values on this standard50,51. All samples and procedural standards were measured at least twice. Sample isotope values are reported as the mean of duplicate or triplicate measurements (Supplementary Table 1). Isotopic values obtained on the NIST 3133 procedural standards are within error of our bracketing standard with an average δ202Hg of 0.02±0.03‰ and Δ199Hg of −0.01±0.01‰ (2 s.e.m., n=4; Supplementary Table 2). Isotopic values for the NIST 1646a procedural standards are consistent with previously measured values for this standard in our laboratory with an average δ202Hg of −0.90±0.05‰ and Δ199Hg of 0.08±0.01‰ (2 s.e.m., n=4; Supplementary Table 2)50. We chose NIST 1646a as a procedural isotope standard for this study because it has Hg concentrations and slight MIF similar to our samples. Over the same time period in the lab, NIST 1944 was also measured by the above combustion procedure and had an average δ202Hg of −0.43±0.03‰ and Δ199Hg of 0.01±0.02‰ (2 s.e.m., n=5), which is within error of published values50. Sample errors are reported as 2 s.e.m. of sample replicates unless that value is smaller than 2 s.d. of the in-house JT Baker Hg standard. If the 2 s.e.m. of sample replicates is smaller than the 2 s.d. of the JT Baker standard, then the 2 s.d. of the JT Baker standard is used as the error for the sample.