Surface Sedimentary Black Carbon Concentrations, Fluxes, and Stable and Radiocarbon Isotopes in the Equatorial Atlantic Ocean

Abstract

Surface sediments (0-1 cm) obtained from equatorial Atlantic Ocean isolated for black carbon using the chemothermal oxidation at 375 method. Multicores were taken during aboard the R.V. Endeavor (EN651) from February 27th 2020 through March 17th 2020 using a MC-800.

Core collection

MC-800 tubes were labeled (EN651-“Site number”-MC”coring attempt number”“letter of core”,ex: EN651-01-MC01a) and photographed before sectioning. The water on top of the core was syphoned off and a thin piece of stainless-steel sheet was slid under the foot of the tube. The foot was bent up and the stainless-steel sheet was used to transfer the core to the core extruder. Cores were sectioned at 1 cm intervals down to 10 cm, then 2 cm intervals down to 20 cm, using the piece of stainless-steel and a cake spatula to cut them. The remainder of the core was wrapped in combusted aluminum foil and placed in a zip-lock bag for storage. Sections of cores were stored in amber glass jars placed in a freezer. One core was transferred with the extruder to a PVC tube and capped for archival storage. If 5 or more cores were recovered, 0.5 cm sections would be taken down to 10 cm and the remainder of the core wrapped in foil and zip-lock bagged before being frozen. Due to a limited supply of jars, the 0.5 cm core sections were wrapped in combusted aluminum foil and placed in a ziplock bag before being stored with other samples. All cores and core sections were stored at -20 ̊C.

Analytical methodsSurface sediment samples (0 – 1 cm) were dried at 60 ˚C until dry and passed through a 420 µm sieve before analysis. Total organic carbon samples were weighed into silver capsules (Elemental microanalysis silver capsules ultra-clean pressed 8 x 5 mm, D2030), acidified to remove inorganic carbon (2 M HCl), and folded into tin capsules (Costech tin capsules 10 x 10 mm, 041073). Black carbon was isolated using the CTO 375 method 34. 100 mg of samples where weighed out into ceramic crucibles and spread into a thin layer prevent charring. Sample were combusted at 375 ˚C for 24 hrs. under the flow of ultra high purity air (0.4 L min-1). The remaining sediment was transferred to GC vials for storage, then processed the same as the TOC samples to remove any inorganic carbon present (as detailed above).

Sampling equipment

Sediment cores were collected using an MC-800

Analytical instrumentation

 An Elemental Analyzer (Costech 4010 Elemental Analyzer) was used for quantification of the BC and TOC fractions. The same elemental analyzer coupled to an Isotope Ratio Mass Spectrometer (Thermo Delta V Advantage) was used for the sample carbon isotopes.  Radiocarbon isotopes were measured at the National Ocean Sciences Accelerator Mass spectrometry.

Parameter names, descriptions, units

Name, "Name of the sediment core from which the top 1 cm was sectioned"Collection Date, "Date the multicore was collected, month/day/yearLat, "Latitude of sampling site", decimal degreesLon, "Longitude of sampling site", decimal degreesDepth, "Water depth of sample site", meters (m)MAR, "Mass accumulation rate", grams per square centimeters per thousand years (g cm-2 kyr-1), blank = no dataTOC, "Total organic carbon concentration", milligrams per gram dry weight (mg g-1)TOC d13C, "Total organic carbon δ13C value", per mill (‰)TOC D14C, "∆14C value of TOC calibrated for a reservoir age of 550 years", per mill (‰), blank = no dataBC, "Black carbon concentration", milligrams per gram dry weight (mg g-1)BC sd, "Black carbon concentration standard deviation", milligrams per gram dry weight (mg g-1)BC d13C, "Black carbon delta 13C value", per mill (‰), NABC D14C, "∆14C value of the BC", per mill (‰), blank = no dataBC flux, "Flux of black carbon to sediments", milligrams per square centimeters per thousand years (mg cm2 kyr-1), blank = no dataBC flux sd, "The standard devation of the flux of black carbon to sediments", milligrams per square centimeters per thousand years (mg cm2 kyr-1), blank = no data