Dataset: Radiocarbon dating of Southeast Pacific sediment cores

Name: Radiocarbon dating of archived sediment cores in the Southeast Pacific from 1960 to 2000
Published: 2023-01-12
Keywords: sediment push cores, radiocarbon dating, Southeast Pacific, oceans

Cite This Dataset

DOI:10.26008/1912/bco-dmo.886679.1

Spatial Extent: N:-17 E:-73.608 S:-44.55 W:-120.193

Temporal Extent: 1960 - 2000

Project:

Collaborative Research: Did the SE Pacific Gyre become a Hot Spot for N2 Fixation during Dusty Glacial Conditions? (N2 Fixation Glacial Dust Pacific Gyre)

Principal Investigator:

Mark A. Altabet (University of Massachusetts Dartmouth, UMass Dartmouth)

Contact:

Mark A. Altabet (University of Massachusetts Dartmouth, UMass Dartmouth)

BCO-DMO Data Manager:

Taylor Heyl (Woods Hole Oceanographic Institution, WHOI BCO-DMO)

Version:

Version Date:

2023-01-12

Restricted:

Validated:

Yes

Current State:

Final no updates expected

Radiocarbon dating of archived sediment cores in the Southeast Pacific from 1960 to 2000

Abstract:

There were likely large changes in Southeast (SE) Pacific Ocean biogeochemistry over the last glacial cycle as a consequence of coincident changes in dust flux, oxygenation, and latitudinal position of the Subantarctic and Antarctic Fronts. However, there are few available sediment core records with reliable chronologies for this time interval despite the large number of archived cores collected between the 1960s and 1990s. The apparent reason is that, except for sites in proximity to the South American continental margin, sedimentation rates are often lower than 1 cm/kyr and calcium carbonate is only well preserved on topographic highs. As part of a project to study past nitrogen fixation, we surveyed archived cores from the SE Pacific and selected the most promising for radiocarbon dating of the planktonic foraminiferal fraction. While many cores have core tops were found to be older than detectable with radiocarbon, a number have core tops ages within the Holocene and sediment accumulation rates centering on 1.5 cm/kyr.

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Archival Copy

Version Date	Archive	Persistent Identifier	Date Assigned
2023-01-12	Marine Biological Laboratory/Woods Hole Oceanographic Institution Library (MBLWHOI DLA)	10.26008/1912/bco-dmo.886679.1	2023-01-17

Methods & Sampling

This dataset was developed for sediment cores from the SE Pacific that were archived in a variety of repositories. The most promising cores were selected for radiocarbon dating of the planktonic foraminiferal fraction. Sediment handling and processing were similar to Cleaveland and Herbert (2007). Sediment samples of 2 cc size were requested from the core libraries where the target cores have been archived. Upon delivery, they were kept refrigerated. Bulk calcium carbonate for radiocarbon dating was isolated from the greater than 150-millimeter (mm) fraction of wet-sieved bulk sediment and dried prior to analysis.

Samples were sent to the WHOI NOSAMS facility and were prepared according to the methods for carbonate 14C analysis listed at https://www2.whoi.edu/site/nosams/client-services/submitting_guidelines/sample_processes (see related publications).

Carbonate samples are treated with phosphoric acid to generate CO2 which, in turn, is converted to a graphite target. The target is analyzed for 14C on an accelerator mass spectrometer (AMS).

Carbonate samples are treated with phosphoric acid while under vacuum. The acidification of the sample material generates carbon dioxide (CO2) which is then converted to graphite. The graphite is pressed into a target to be analyzed for 14C on an accelerator mass spectrometer (AMS).

Data Processing Description

Data processing including error estimates are as in https://www2.whoi.edu/site/nosams/client-services/radiocarbon-data-calculations/ and as in Stuvier and Polach (1977).

In short, the fraction of 14C expected for modern samples (FM) is calculated from the AMS-measured 14C/12C ratio. The radiocarbon age is then calculated from the radioactive decay equation and decay constant for 14C. Where warranted corrections were made by NOSAMS for background 14C.

BCO-DMO Processing Description:
- Adjusted field/parameter names to comply with BCO-DMO naming conventions
- Added a conventional header with dataset name, PI names, version date

Data Files

File
14c_dates.csv (Comma Separated Values (.csv), 3.00 KB) MD5:543a23d86332f34a2d05414915f9b767 Primary data file for dataset ID 886679

More Information about this dataset

Funding Sources

Funding Source	Award Number
NSF Division of Ocean Sciences	OCE-1602810

Deployments

None available yet

Instruments

Accelerator Mass Spectrometer

Supplied Name: WHOI NOSAMS Accelerator Mass Spectrometer

Supplied Description:

The WHOI NOSAMS Accelerator Mass Spectrometer is as described in https://www2.whoi.edu/site/nosams/about/laboratory-capabilities/ams-instruments/

Instrument Type

Generic Name: Accelerator Mass Spectrometer

Acronym: AMS

Community Identifier: http://vocab.nerc.ac.uk/collection/L05/current/LAB17/

Generic Description:

An AMS measures "long-lived radionuclides that occur naturally in our environment. AMS uses a particle accelerator in conjunction with ion sources, large magnets, and detectors to separate out interferences and count single atoms in the presence of 1x1015 (a thousand million million) stable atoms, measuring the mass-to-charge ratio of the products of sample molecule disassociation, atom ionization and ion acceleration." AMS permits ultra low-level measurement of compound concentrations and isotope ratios that traditional alpha-spectrometry cannot provide. More from Purdue University: http://www.physics.purdue.edu/primelab/introduction/ams.html

Parameters

Date and time formats are described in python strftime() syntax.

Supplied Name	Supplied description	Supplied Units	Standard Name
Depth_in_core	Depth below core top from which the sample was taken	centimeters (cm)	depth_core
Core_Longitude	Longitude of the core site East (West is negative)	decimal degrees	lon
Core_latitude	Latitude of the core site North (South is negative)	decimal degrees	lat
Measured_Fraction_Modern	Fraction of modern radiocarbon in the sample as measured by the AMS	unitless	14C
Measured_Fraction_Modern_Error	± Error in fraction of modern radiocarbon determined by NOSAMS	unitless	std_err
Radiocarbon_Age	Radiocarbon age derived from fraction of modern radiocarbon. Where a '>' is shown, statistical certainty permitted only a lower bound to be determined.	unitless	C14_age
Radiocarbon_Age_Error	Age error determined from the error in fraction of modern radiocarbon, in years. Where age was only lower bounded no error is calculated.	unitless	std_err
Sediment_Core_ID	Core designation at the time of collection and used by the core library where it is archived. All cores were piston cores except when a trigger core was available and then 'PC' or 'TC' is added to the designation. 'replicate' indicates a replicate analysis from the same sediment sample	unitless	core_id

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Database

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Dataset: Radiocarbon dating of Southeast Pacific sediment cores