Benthic iron fluxes from sites along the Oregon–California continental shelf determined using in situ benthic chambers, range from less than 10
μmol
m
−2
d
−1 to values in excess of ∼300
μmol
m
−2
d
...−1. These fluxes are generally greater than previously published iron fluxes for continental shelves contiguous with the open ocean (as opposed to marginal seas, bays, or estuaries) with the highest fluxes measured in the regions around the high-sediment discharge Eel River and the Umpqua River. These benthic iron fluxes do not covary with organic carbon oxidation rates in any systematic fashion, but rather seem to respond to variations in bottom water oxygen and benthic oxygen demand. We hypothesize that the highest rates of benthic iron efflux are driven, in part, by the greater availability of reactive iron deposited along these river systems as compared to other more typical continental margin settings. Bioirrigation likely plays an important role in the benthic Fe flux in these systems as well. However, the influence of bottom water oxygen concentrations on the iron flux is significant, and there appears to be a threshold in dissolved oxygen (∼60–80
μM), below which sediment–ocean iron exchange is enhanced. The isotope composition of this shelf-derived benthic iron is enriched in the lighter isotopes, and appears to change by ∼3‰ (δ
56Fe) during the course of a benthic chamber experiment with a mean isotope composition of −2.7
±
1.1‰ (2 SD,
n
=
9) by the end of the experiment. This average value is slightly heavier than those from two high benthic Fe flux restricted basins from the California Borderland region where δ
56Fe is −3.4
±
0.4‰ (2 SD,
n
=
3). These light iron isotope compositions support previous ideas, based on sediment porewater analyses, suggesting that sedimentary iron reduction fractionates iron isotopes and produces an isotopically light iron pool that is transferred to the ocean water column. In sum, our data suggest that continental shelves may export a higher efflux of iron than previously hypothesized, with the likelihood that along river-dominated margins, the benthic iron flux could well be orders of magnitude larger than non-river dominated shelves. The close proximity of the continental shelf benthos to the productive surface ocean means that this flux is likely to be essential for maintaining ecosystem micronutrient supply.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This study examines causes of the double silica maximum in the deep interior Northeast Pacific Basin using a stochastic Lagrangian tracer model based on steady‐state advective fields and diapycnal ...diffusion established by a hydrographic inverse method that conserves potential vorticity and salinity. Lateral diffusion, unresolved by the inverse model, is adjusted for overall agreement with radiocarbon distribution. The double silica maximum in vertical profiles arises from an eastern‐intensified single maximum in the North Pacific Deep Water along the northern domain boundary (originating in the western Pacific), and a strong subarctic bottom source supplying silica to Upper Circumpolar Deep Water density surfaces that successively intersect the seafloor over a broad area east of 150°W, associated geostrophically with southward flow. The existence of the double silica maximum requires weak diapycnal transport in the deep interior, with broader implications for the conceptual picture of meridional overturning circulation in the North Pacific.
Plain Language Summary
Silica, an important nutrient supporting diatom production, has two distinct vertical concentration maxima in the deep (>1,000 m) northeast Pacific ocean. This structure suggests that distinct processes create each of these features. In this study, we explore how large‐scale deep ocean circulation, quantified by a recent study, combines with a simple latitude‐dependent source of silica from seafloor sediments to create a near‐bottom maximum within a southward current in the densest layers in the eastern half of the basin. In contrast, a shallower (middepth) silica maximum in the vertical structure of concentration profiles is already present in the inflowing deep water and is not substantially modified as it flows through this part of the ocean.
Key Points
An upper (North Pacific Deep Water) silica maximum is advecting through this region with only minor modification
A lower (Upper Circumpolar Deep Water) maximum is created locally by density structure, flow pathways, and enhanced subarctic seafloor flux
The existence of the double silica maximum is linked to weak diapycnal mixing, with broader implications for North Pacific overturning
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Radium‐226(226Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite ...their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full‐depth ocean section of 226Ra activities (T1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of 226Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher 226Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with 226Ra inputs primarily resulting from the decay of 230Th in sediments. Further, dissolved 226Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non‐conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non‐conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of 226Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non‐conservative processes that shape their respective distributions.
Key Points
Water mass analysis shows significant shallow deficits and deep excesses of dissolved 226Ra and Ba in the Pacific Ocean along the GP15
Shallow deficits seem driven by BaSO4 precipitation, deep excesses reflect benthic inputs that are more significant for 226Ra than for Ba
Budgets suggest that regional barium fluxes nearly balance, and the Pacific is likely a significant source of radium‐226 to the global ocean
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Ocean disposal of industrial waste from technical DDT mainly 1,1′-(2,2,2-trichloroethane-1,1-diyl)bis(4-chlorobenzene), or 4,4′-DDT manufacture occurred historically in the Southern California ...Bight. However, the paucity of historical records highlights uncertainties as to the mode, location, and timing of disposal or ongoing ecological effects of these wastes. This study combines sampling, chemical analysis, and numerical modeling of deep San Pedro Basin sediments revealing substantial DDT contamination that extends at least 25 km from the mainland. These findings narrate bulk DDT waste disposal to the offshore that peaked in the 1950s, prior to the onset of formal regulations; was agnostic to later-designated disposal sites; and has experienced sluggish transformation. Our findings further indicate an attenuating secondary source for the DDT daughter product, 1-chloro-4-2,2-dichloro-1-(4-chlorophenyl)ethenylbenzene (4,4′-DDE), which still deposits into deep San Pedro Basin sediments. While demonstrating the severity of DDT contamination to the region, these findings further define the burial potential of DDT wastes and inform the past, present, and future contamination potential that is needed to understand and predict ecological consequences. This work also points firmly to bulk, not containerized, disposal of DDT waste and to potential alternative contents of collocated waste.
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IJS, KILJ, NUK, PNG, UL, UM
Sea-level rise is particularly concerning for tidal wetlands that reside within an area with steep topography or are constrained by human development and alteration of sedimentation. Sediment ...augmentation to increase wetland elevations has been considered as a potential strategy for such areas to prevent wetland loss over the coming decades. However, there is little information on the best approaches and whether adaptive management actions can mimic natural processes to build sea-level rise resilience. In addition, the lack of information on long-term marsh characteristics, processes, and variability can hamper development of effective augmentation strategies. Here, we assess a case study in a southern California marsh to determine the nature of the pre-existing sediments and variability of the site in relation to sediments applied during an augmentation experiment. Although sediment cores revealed natural variations in the grain size and organic content of sediments deposited at the site over the past 1500 years, the applied sediments were markedly coarser in grain size than prehistoric sediments at the site (100% maximum sand versus 76% maximum sand). The rate of the experimental sediment application (25.1 ± 1.09 cm in ~2 months) was also much more rapid than natural accretion rates measured for the site historically. In contrast, post-augmentation sediment accretion rates on the augmentation site have been markedly slower than pre-augmentation rates or current rates on a nearby control site. The mismatch between the characteristics of the applied sediment and thickness of application and the historic conditions are likely strong contributors to the slow initial recovery of vegetation. Sediment augmentation has been shown to be a useful strategy in some marshes, but this case study illustrates that vegetation recovery may be slow if applied sediments are not similar or at a thickness similar to historic conditions. However, testing adaptation strategies to build wetland elevations is important given the long-term risk of habitat loss with sea-level rise. Lessons learned in the case study could be applied elsewhere.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Chemical weathering of volcanic rocks in warm and humid climates contributes disproportionately to global solute fluxes. Geochemical signatures of solutes and solids formed during this process can ...help quantify and reconstruct weathering intensity in the past. Here, we measured silicon (Si) and germanium (Ge) isotope ratios of the soils, clays, and fluids from a tropical lowland rainforest in Costa Rica. The bulk topsoil is intensely weathered and isotopically light (mean ± 1σ: δ30Si = −2.1 ± 0.3‰, δ74Ge = −0.13 ± 0.12‰) compared to the parent rock (δ30Si = −0.11 ± 0.05‰, δ74Ge = 0.59 ± 0.07‰). Neoforming clays have even lower values (δ30Si = −2.5 ± 0.2‰, δ74Ge = −0.16 ± 0.09‰), demonstrating a whole‐system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (δ30Si = 0.2 − 0.6‰, δ74Ge = 2.2 − 2.6‰) with solute‐rich interbasin groundwater (δ30Si = 1.0 ± 0.2‰, δ74Ge = 4.0‰). Using a Ge‐Si isotope mass balance model, we calculate that 91 ± 9% of Ge released via weathering of lowland soils is sequestered by neoforming clays, 9 ± 9% by vegetation, and only 0.2 ± 0.2% remains dissolved. Vegetation plays an important role in the Si cycle, directly sequestering 39 ± 14% of released Si and enhancing clay neoformation in surface soils via the addition of amorphous phytolith silica. Globally, volcanic soil δ74Ge closely tracks the depletion of Ge by chemical weathering (τGe), whereas δ30Si and Ge/Si both reflect the loss of Si (τSi). Because of the different chemical mobilities of Ge and Si, a δ74Ge‐δ30Si multiproxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation.
Key Points
Very low Ge and Si isotope ratios in clays and bulk soils reflect intense chemical weathering
Si cycling is strongly influenced by vegetation in extremely weathered soils
In global volcanic soils, Si isotopes and Ge/Si reflect Si loss, while Ge isotopes reflect Ge loss
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and timing relationships to particular concretions has ...remained elusive. Previously employed proxies (e.g., carbon and oxygen isotopes) cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen, nitrate, metal oxides, and sulfate as electron acceptors, in addition to degradation by thermal decarboxylation. Here, we employ concentrations of carbonate-associated sulfate (CAS) and δ
34S
CAS (along with more traditional approaches) to determine the specific nature of concretion authigenesis within the Miocene Monterey Formation.
Integrated geochemical analyses reveal that at least three specific organo-diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample-site specific. One calcitic concretion from the Phosphatic Shale Member at Naples Beach yields δ
34S
CAS values near Miocene seawater sulfate (∼+22‰ VCDT), abundant CAS (ca. 1000
ppm), depleted δ
13C
carb (∼−11‰ VPDB), and very low concentrations of Fe (ca. 700
ppm) and Mn (ca. 15
ppm)—characteristics most consistent with shallow formation in association with organic matter degradation by nitrate, iron-oxides and/or minor sulfate reduction. Cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated δ
34S
CAS (up to ∼+37‰), CAS concentrations of ∼600
ppm, mildly depleted δ
13C
carb (∼−6‰), moderate amounts of Mn (ca. 250
ppm), and relatively low Fe (ca. 1700
ppm), indicative of formation in sediments dominated by sulfate reduction. Finally, concretions within a siliceous host at Montaña de Oro and Naples Beach show minimal CAS concentrations, positive δ
13C values, and the highest concentrations of Fe (ca. 11,300
ppm) and Mn (ca. 440
ppm), consistent with formation in sediments experiencing methanogenesis in a highly reducing environment. This study highlights the promise in combining CAS analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Diatoms are the dominant algal group that cycles dissolved silicic acid in the ocean; they also play an important role in the oceanic carbon cycle. It is therefore important to quantify the spatial ...distribution of silica cycling for defining global ocean biogeochemical cycles. On the research cruise CDisK-IV, water samples and sediment cores were collected at 5 stations along a North Pacific transect near 150°W from 22°N to 50°N to evaluate benthic remineralization rates of biogenic silica (bSi). Two independent methods, core incubation and diffusive transport based on porewater profiles, were utilized to estimate benthic silicic acid fluxes, and these independent methods yield fluxes that agree within uncertainties. The benthic fluxes are reported as 0.04 ± 0.01, 0.04 ± 0.01, 0.05 ± 0.01, 0.67 ± 0.14, 0.40 ± 0.08 mmol Si m−2 day−1 for Stations 1 to 5, south to north, respectively. Burial fluxes were estimated using measurements of solid phase bSi in sediments and literature values of sediment accumulation rate. Burial efficiencies of bSi at all stations were <5% and show reasonable agreement with previous estimates. When burial rates were added to benthic fluxes to calculate rain rates, the rain observed under the subarctic gyre (Stations 4–5), was far larger than in the lower latitudes of the subtropics (Stations 1–3), corresponding to higher surface diatom productivity at higher latitudes. At the two northern stations, the bottom 500 m of the water column shows a near-bottom increase in silicic acid that is consistent with the measured benthic flux and the estimated vertical eddy diffusivity. Above this horizon, water column density stratification increases and vertical diffusivity decreases, but the silicic acid gradient decreases. This reduction in gradient indicates that above this horizon, horizontal transport by deep waters, rather than vertical diffusion, becomes the dominant process removing the silicic acid released by benthic remineralization.
•Benthic Si fluxes in the North Pacific were estimated with two independent methods.•Fluxes were 10 times higher in the subarctic gyre than in the subtropical gyre.•Burial efficiency of biogenic silica was low at all study sites.•The spatial pattern of benthic Si fluxes was similar to that of upper ocean chlorophyll.•The benthic flux was sufficient to sustain the near-bottom Si increase in the water column.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this study we have sampled the water column and sediments of the Gulf of Mexico to investigate the effects of high riverine terrigenous load and sediment redox conditions on the cycling of Ge and ...Si. Water column Ge/Si ratios across the Gulf of Mexico continental shelf range from 1.9 to 25μmol/mol, which is elevated compared to the global ocean value of 0.7μmol/mol. The Ge enrichment in the Gulf of Mexico seawater is primarily due to anthropogenic contamination of the Mississippi river, which is the main Ge and Si source to the area, and to a smaller extent due to discrimination against Ge during biogenic silica (bSi) production (Ge/Si=1.2–1.8μmol/mol), especially by radiolarians and siliceous sponges (Ge/Si=0.6–1.1μmol/mol). Most sediment pore waters (Ge/Si=0.3–4.5μmol/mol) and sediment incubation experiments (benthic flux Ge/Si=0.9–1.2μmol/mol) indicate precipitation of authigenic phases that sequester Ge from pore waters (non-opal sink). This process appears to be independent of oxidation–reduction reactions and suggests that authigenic aluminosilicate formation (reverse weathering) may be the dominant Ge sink in marine sediments. Compilation of previously published data shows that in continental margins, non-opal Ge burial flux is controlled by bSi supply, while in open ocean sediments it is 10–100 times lower and most likely limited by the supply of lithogenic material. We provide a measurement-based estimate of the global non-opal Ge burial flux as 4–32Mmolyr−1, encompassing the 2–16Mmolyr−1 needed to keep the global marine Ge cycle at steady state.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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