The mean age of ocean waters inferred from radiocarbon observations : sensitivity to surface sources and accounting for mixing histories

Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 42 (2012): 291–305, doi:10.1175/JPO-D-11-043.1.A number of previous observational studies have found that the waters of the deep Pacific Ocean have an age, or elapsed time since contact with the surface, of 700–1000 yr. Numerical models suggest ages twice as old. Here, the authors present an inverse framework to determine the mean age and its upper and lower bounds given Global Ocean Data Analysis Project (GLODAP) radiocarbon observations, and they show that the potential range of ages increases with the number of constituents or sources that are included in the analysis. The inversion requires decomposing the World Ocean into source waters, which is obtained here using the total matrix intercomparison (TMI) method at up to 2° × 2° horizontal resolution with 11 113 surface sources. The authors find that the North Pacific at 2500-m depth can be no younger than 1100 yr old, which is older than some previous observational estimates. Accounting for the broadness of surface regions where waters originate leads to a reservoir-age correction of almost 100 yr smaller than would be estimated with a two or three water-mass decomposition and explains some of the discrepancy with previous observational studies. A best estimate of mean age is also presented using the mixing history along circulation pathways. Subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200–1500 yr old, which is more in line with existing numerical model results.GG is supported by the J. Lamar Worzel Assistant Scientist Fund and the Penzance Endowed Fund in Support of Assistant Scientists. PJH is supported by NSF Award 0960787.2012-08-0

Can paleoceanographic tracers constrain meridional circulation rates?

Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 37 (2007): 394-407, doi:10.1175/jpo3018.1.The ability of paleoceanographic tracers to constrain rates of transport is examined using an inverse method to combine idealized observations with a geostrophic model. Considered are the spatial distribution, accuracy, and types of tracers required to constrain changes in meridional transport within an idealized single-hemisphere basin. Measurements of density and radioactive tracers each act to constrain rates of transport. Conservative tracers, while not of themselves able to inform regarding rates of transport, improve constraints when coupled with density or radioactive observations. It is found that the tracer data would require an accuracy one order of magnitude better than is presently available for paleo-observations to conclusively rule out factor-of-2 changes in meridional transport, even when assumed available over the entire model domain. When data are available only at the margins and bottom of the model, radiocarbon is unable to constrain transport while density remains effective only when a reference velocity level is assumed. The difficulty in constraining the circulation in this idealized model indicates that placing firm bounds on past meridional transport rates will prove challenging.The first author is supported by the NOAA Postdoctoral Program in Climate and Global Change and GG by the National Ocean Partnership Program (ECCO). Author OM acknowledges support from the National Science Foundation

The SO4:Cl Ratio in Oceanic Rainwater

Rainwater samples collected on 20 islands in the Pacific and Atlantic oceans since 1961 have been analyzed for dissolved sulfate and chloride ion concentrations. Samples from sparsely populated islands have S04: CI ratios approximately equal to that observed in seawater. Notable exceptions are samples from Midway and Christmas islands, which are enriched in sulfate, and those from the ocean stations Echo, Papa, and Victor, which are depleted in sulfate. The 8180 of the oxygen in the S04 ion ranges from 9.5 %0 in seawater and unpolluted rain to 11.2 %0 in rain derived from a continental air mass. Aerosols and snow samples have higher 8180 values of 11.2 to 14.8%0 due to their greater efficiency in trapping locally produced sulfate pollutants

Geochemistry and isotopes at DSDP Leg 80 Holes

Thirty-eight samples from DSDP Sites 549 to 551 were analyzed for major and minor components and trace element abundances. Multivariate statistical analysis of geochemical data groups the samples into two major classes: an organic-carbon- rich group (> 1% TOC) containing high levels of marine organic matter and certain trace elements (Cu, Zn, V, Ni, Co, Ba, and Cr) and an organic-carbon-lean group depleted in these components. The greatest organic and trace metal enrichments occur in the uppermost Albian to Turanian sections of Sites 549 to 551. Carbon-isotopic values of bulk carbonate for the middle Cenomanian section of Site 550 (2.35 to 2.70 per mil) and the upper Cenomanian-Turonian sections of Sites 549 (3.35 to 4.47 per mil) and 551 (3.13 to 3.72 per mil) are similar to coeval values reported elsewhere in the region. The relatively heavy d13C values from Sites 549 and 551 indicate that this interval was deposited during the global "oceanic anoxic event" that occurred at the Cenomanian/Turonian boundary. Variation in the d18O of bulk carbonate for Section 550B-18-1 of middle Cenomanian age suggests that paleosalinity and/or paleotemperature variations may have occurred concurrently with periodic anoxia at this site. Climatically controlled increases in surface-water runoff may have caused surface waters to periodically freshen, resulting in stable salinity stratificatio

Stable isotope ratios of benthic and planktonic foraminifera from Miocene to Pliocene sediments of DSDP Hole 7-62A on the Eauripik Rise, Pacific Ocean (Table 1)

Oxygen and carbon isotope analyses show that the biserial forarniniferal genus Streptochilus, which was originally described from pelagic sediments on the Eauripik Rise and Ontong Java Plateau, lived deep in the upper water column within the oxygen minimum layer. The species of Streptochilus average from 4 to 19% of the foraminiferal assemblages in which benthic forms compose less than 1 or 2%. Specimens of Streptochilus are selectively dissolved when in contact with the bottom water mass. Their rapid evolutionary turnover of less than a few million years and their wide areal distribution in the equatorial Indo-Pacific are indicative of planktonic foraminifera. Aside from usefulness of the species of Streptochilus as stratigraphic indices, these Neogene biserial planktonic foraminifera are potential indices of paleoceanographic stratification

Isotopes and gases in a Hawaiian geothermal system : HGP-A

Discussion of early isotopic analyses conducted on the fluids from the HGP-A well