skip to main content

Attention:

The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 11:00 PM ET on Thursday, May 23 until 2:00 AM ET on Friday, May 24 due to maintenance. We apologize for the inconvenience.


Search for: All records

Creators/Authors contains: "Landing, William M."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract

    We use a tracer method involving the cosmogenic radioisotope beryllium‐7 (half‐life = 53.3 days) to follow the deposition of aerosols and the fate of snow on the MOSAiC ice floe during winter and spring 2019–2020. When examined alongside data from earlier studies in the Arctic Ocean that covered summer and fall, Be‐7 inventories indicate a summertime peak for aerosol Be‐7 deposition fluxes coinciding with seasonal minima boundary‐level aerosol concentrations, which suggests that deposition fluxes are primarily controlled by precipitation. This conclusion is supported by the linear relationship between Be‐7 fluxes and precipitation rates derived from data from the MOSAiC and SHEBA expeditions. Inventories of Be‐7 within the snow column exhibited evidence of significant redistribution. Be‐7 deficits, relative to the flux, were observed in areas of level sea ice while excess Be‐7 was found associated with deformed ice features such as pressure ridges, leading to the following estimates for the distribution of snow on the ice floe in May 2020: 75–93% of the snow mass is found on deformed sea ice with the remainder on level ice. Furthermore, uncertainties associated with measurements of Be‐7 concentrations within the ocean mixed layer would allow for losses of snow through open leads of up to approximately 20% of the flux. Our snow distribution estimates agree with data from repeat snow depth transect measurements. These results suggest that Be‐7 can be a useful tool in studying snow redistribution.

     
    more » « less
    Free, publicly-accessible full text available February 21, 2025
  2. Abstract

    Atmospheric deposition of aerosols transported from the continents is an important source of nutrient and pollutant trace elements (TEs) to the surface ocean. During the U.S. GEOTRACES GP15 Pacific Meridional Transect between Alaska and Tahiti (September–November 2018), aerosol samples were collected over the North Pacific and equatorial Pacific and analyzed for a suite of TEs, including Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb. Sampling coincided with the annual minimum in dust transport from Asia, providing an opportunity to quantify aerosol TE concentrations and deposition during the low dust season. Nevertheless, peak concentrations of “crustal” TEs measured at ∼40–50°N (∼145 pmol/m3Fe) were associated with transport from northern Asia, with lower concentrations (36 ± 14 pmol/m3Fe) over the equatorial Pacific. Relative to crustal abundances, equatorial Pacific aerosols typically had higher TE enrichment factors than North Pacific aerosols. In contrast, aerosol V was more enriched over the North Pacific, presumably due to greater supply to this region from oil combustion products. Bulk deposition velocity (Vbulk) was calculated along the transect using the surface ocean decay inventory of the naturally occurring radionuclide,7Be, and aerosol7Be activity. Deposition velocities were significantly higher (4,570 ± 1,146 m/d) within the Intertropical Convergence Zone than elsewhere (1,764 ± 261 m/d) due to aerosol scavenging by intense rainfall. Daily deposition fluxes to the central Pacific during the low dust season were calculated using Vbulkand aerosol TE concentration data, with Fe fluxes ranging from 19 to 258 nmol/m2/d.

     
    more » « less
  3. Abstract

    Despite the Pacific being the location of the earliest seawater Cd studies, the processes which control Cd distributions in this region remain incompletely understood, largely due to the sparsity of data. Here, we present dissolved Cd and δ114Cd data from the US GEOTRACES GP15 meridional transect along 152°W from the Alaskan margin to the equatorial Pacific. Our examination of this region's surface ocean Cd isotope systematics is consistent with previous observations, showing a stark disparity between northern Cd‐rich high‐nutrient low‐chlorophyll waters and Cd‐depleted waters of the subtropical and equatorial Pacific. Away from the margin, an open system model ably describes data in Cd‐depleted surface waters, but atmospheric inputs of isotopically light Cd likely play an important role in setting surface Cd isotope ratios (δ114Cd) at the lowest Cd concentrations. Below the surface, Southern Ocean processes and water mass mixing are the dominant control on Pacific Cd and δ114Cd distributions. Cd‐depleted Antarctic Intermediate Water has a far‐reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern‐sourced Cd signatures in North Pacific Intermediate Water. Finally, we show that the previously identified negative Cd* signal at depth in the North Pacific is associated with the PO4maximum and is thus a consequence of an integrated regeneration signal of Cd and PO4at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol−1), rather than being related to in situ removal processes in low‐oxygen waters.

     
    more » « less
  4. Abstract

    Deposition of aerosols to the surface ocean is an important factor affecting primary production in the surface ocean. However, the sources and fluxes of aerosols and associated trace elements remain poorly defined. Aerosol210Pb,210Po, and7Be data were collected on US GEOTRACES cruise GP15 (Pacific Meridional Transect, 152°W; 2018).210Pb fluxes are low close to the Alaskan margin, increase to a maximum at ∼43°N, then decrease to lower values. There is good agreement between210Pb fluxes and long‐term land‐based fluxes during the SEAREX program (1970–1980s), as well as between GP15 and GP16 (East Pacific Zonal Transect, 12°S; 2013) at adjacent stations. A normalized fractionf(7Be,210Pb) is used to discern aerosols with upper (highf) versus lower (lowf) troposphere sources. Alaskan/North Pacific aerosols show significant continental influence while equatorial/South Pacific aerosols are supplied to the marine boundary layer from the upper troposphere. Lithogenic trace elements Al and Ti show inverse correlations withf(7Be,210Pb), supporting a continental boundary layer provenance while anthropogenic Pb shows no clear relationship withf(7Be,210Pb). All but four samples have210Po/210Pb activity ratios <0.2 suggesting short aerosol residence time. Among the four samples (210Po/210Pb = 0.42–0.88), two suggest an upper troposphere source and longer aerosol residence time while the remaining two cannot be explained by long aerosol residence time nor a significant component of dust. We hypothesize that enrichments of210Po in them are linked to Po enrichments in the sea surface microlayer, possibly through Po speciation as a dissolved organic or dimethyl polonide species.

     
    more » « less
  5. Abstract

    Atmospheric dust is an important source of the micronutrient Fe to the oceans. Although relatively insoluble mineral Fe is assumed to be the most important component of dust, a relatively small yet highly soluble anthropogenic component may also be significant. However, quantifying the importance of anthropogenic Fe to the global oceans requires a tracer which can be used to identify and constrain anthropogenic aerosols in situ. Here, we present Fe isotope (δ56Fe) data from North Atlantic aerosol samples from the GEOTRACES GA03 section. While soluble aerosol samples collected near the Sahara have near-crustal δ56Fe, soluble aerosols from near North America and Europe instead have remarkably fractionated δ56Fe values (as light as −1.6‰). Here, we use these observations to fingerprint anthropogenic combustion sources, and to refine aerosol deposition modeling. We show that soluble anthropogenic aerosol Fe flux to the global surface oceans is highly likely to be underestimated, even in the dusty North Atlantic.

     
    more » « less
  6. Abstract

    Atmospheric deposition represents a major input for micronutrient trace elements (TEs) to the surface ocean and is often quantified indirectly through measurements of aerosol TE concentrations. Sea spray aerosol (SSA) dominates aerosol mass concentration over much of the global ocean, but few studies have assessed its contribution to aerosol TE loading, which could result in overestimates of “new” TE inputs. Low‐mineral aerosol concentrations measured during the U.S. GEOTRACES Pacific Meridional Transect (GP15; 152°W, 56°N to 20°S), along with concurrent towfish sampling of surface seawater, provided an opportunity to investigate this aspect of TE biogeochemical cycling. Central Pacific Ocean surface seawater Al, V, Mn, Fe, Co, Ni, Cu, Zn, and Pb concentrations were combined with aerosol Na data to calculate a “recycled” SSA contribution to aerosol TE loading. Only vanadium was calculated to have a SSA contribution averaging >1% along the transect (mean of 1.5%). We derive scaling factors from previous studies on TE enrichments in the sea surface microlayer and in freshly produced SSA to assess the broader potential for SSA contributions to aerosol TE loading. Maximum applied scaling factors suggest that SSA could contribute significantly to the aerosol loading of some elements (notably V, Cu, and Pb), while for others (e.g., Fe and Al), SSA contributions largely remained <1%. Our study highlights that a lack of focused measurements of TEs in SSA limits our ability to quantify this component of marine aerosol loading and the associated potential for overestimating new TE inputs from atmospheric deposition.

     
    more » « less