Methodology:
Approximately 50 to 100 mg of sediment were initially treated with 3 ml of 2 M HNO₃ and heated at 130°C for ~12 h to chemically separate the authigenic and detrital fraction of Tl. This treatment dissolves pyrite, which is thought to contain the authigenic Tl fraction, while the undissolved portion is dominated by silicate minerals. The two sample fractions were then separated via three cycles of centrifugation, pipetting off the supernatant (henceforth termed 'the authigenic fraction') and rinsing in MQ water. After separation of the authigenic and silicate fractions, both fractions were digested separately in the HPA using ~3 ml of concentrated nitric and ~3 ml of concentrated hydrochloric acid in order to break down organic molecules. After the HPA digestion, authigenic fractions were prepared for anion exchange separation of Tl from the sample matrix; no hydrofluoric acid treatment was used on the authigenic fraction, thereby reducing possible Tl contributions from silicates. Residual silicates were dissolved using a hydrofluoric/nitric acid mixture and then prepared for anion exchange separation (henceforth termed 'the silicate fraction') for thallium purification.

Sampling and analytical procedures:
Thallium Purification: Thallium was isolated from a sample matrix using previously described anion exchange chromatography techniques. Briefly, thallium purification was accomplished using two separate columns, one large and one micro-column, using AG1X8 anion exchange resin from Biorad. The two columns quantitatively remove lead (Pb) from the sample, which is required to monitor mass-bias during mass spectrometric analysis. NIST SRM 981 Pb is added to every sample to monitor for instrumental mass bias. In addition, this allows for determination of Tl concentrations during isotopic analysis because the yield of the Tl column chemistry is within error of 100%. However, only one column was used during Tl purification. Testing of standards and samples showed that there was no difference in concentration or isotope composition between samples that had been processed through one or two columns.

Isotope Instrumental Configuration: Samples were analyzed for Tl isotopic compositions using a Thermo Scientific Neptune multi-collector inductively-coupled-plasma mass spectrometer (MC-ICP-MS) located in the WHOI Plasma Facility. Measurement were performed using low resolution with jet sample and Ni X-skimmer cones coupled with a dry plasma inlet system with Aridus II desolvator (CETAC Technologies).

TI_isotopes.csv