Hong Chin Ng

The flux of nutrients from continents to the oceans sustains oceanic primary productivity and is a fundamental component of the carbon cycle. In most regions of the world's oceans primary productivity is limited by the supply of nutrients. In particular, iron can become limiting in the open‐ocean due to its low solubility. Glaciated continents have been suggested as an underappreciated source of iron to the high‐latitude oceans. Yet, uncertainty remains regarding the magnitude and spatial variability of glacially derived nutrient fluxes, and the extent to which these nutrients impact open‐ocean ecosystems. To quantify lithogenic fluxes at the West Greenland margin, we measured ²³² Th and ²³⁰ Th in seawater and core‐top sediments across the shelf and slope. Our results highlight a negative correlation between low‐salinity waters and dissolved and particulate ²³² Th, suggesting a glacial source for this lithogenic isotope. We calculated dissolved ²³² Th fluxes 5–24 μg m ⁻² yr ⁻¹ (100–500 m depth), and sedimentary ²³² Th fluxes 105–711 μg m ⁻² yr ⁻¹ , higher than typical open‐ocean settings and similar to margin sites influenced by large inputs from aeolian dust and rivers. A sampling transect shows that dissolved ²³² Th fluxes increase toward Greenland, confirming that lithogenic inputs are sourced laterally from the margin. Using our ²³² Th fluxes, we estimate an elevated supply of dissolved Fe which extends over the continental slope toward the open ocean. This Fe flux is large enough to support much of the local primary productivity, highlighting the importance of lithogenic fluxes in supporting the marine ecosystem in high‐latitude oceans.

Abundant proxy records suggest a profound reorganization of the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM, ~21,000 y ago), with the North Atlantic Deep Water (NADW) shoaling significantly relative to the present-day (PD) and forming Glacial North Atlantic Intermediate Water (GNAIW). However, almost all previous observational and modeling studies have focused on the zonal mean two-dimensional AMOC feature, while recent progress in the understanding of modern AMOC reveals a more complicated three-dimensional structure, with NADW penetrating from the subpolar North Atlantic to lower latitude through different pathways. Here, combining ²³¹ Pa/ ²³⁰ Th reconstructions and model simulations, we uncover a significant change in the three-dimensional structure of the glacial AMOC. Specifically, the mid-latitude eastern pathway (EP), located east of the Mid-Atlantic Ridge and transporting about half of the PD NADW from the subpolar gyre to the subtropical gyre, experienced substantial intensification during the LGM. A greater portion of the GNAIW was transported in the eastern basin during the LGM compared to NADW at the PD, resulting in opposite ²³¹ Pa/ ²³⁰ Th changes between eastern and western basins during the LGM. Furthermore, in contrast to the wind-steering mechanism of EP at PD, the intensified LGM EP was caused primarily by the rim current forced by the basin-scale open-ocean convection over the subpolar North Atlantic. Our results underscore the importance of accounting for three-dimensional oceanographic changes to achieve more accurate reconstructions of past AMOC.

In the Arctic and subarctic oceans, the relatively low supply of silicon (compared to other nutrients) can make it limiting for the growth of diatoms, a fundamental building block of the oceanic food web. Glaciers release large quantities of dissolved silicon and dissolvable solid amorphous silica phases into high‐latitude estuaries (fjords), but the role of these glacially‐derived silica phases in sustaining diatom growth in the coastal and open‐water sectors remains unknown. Here we show how stable and radiogenic silicon isotopes can be used together to address this question, using southwest Greenland as a case study. This study finds enhanced levels of detrital (i.e., mineral) amorphous silica, likely glacially‐sourced, sustaining a large portion of diatom growth observed off the coast, revealing how the phytoplankton community can function during high‐meltwater periods.

Glacier meltwater supplies a significant amount of silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these essential nutrients reach the ocean is regulated by the processes occurring within fjords, which act as conduits from glacial rivers and subglacial discharge and the ocean. One key – but understudied – component of biogeochemistry within fjords is benthic cycling, especially in regions of rapid deposition of reactive particulates at fjord heads. Here, we explore the benthic cycling of Si and Fe at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including stable Si and Fe isotopes (δ30Si and δ56Fe respectively), and novel reaction-transport modelling for Si. A high diffusive flux of dissolved Fe from the fjord sediments compared to open ocean sediments is supported by both reductive and non-reductive dissolution of glacially-sourced reactive Fe phases, as reflected by the range of pore water stable Fe isotopes (δ56Fe from -2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments is relatively low. High pore water δ30Si (up to +3.3‰) observed near the Fe(II)-Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe but suppresses the release of dissolved Si at glaciated fjord heads. The redox sensitive coupling of Si and Fe has significant implications for our understanding of how essential nutrients are transport along fjord systems.

Glacier meltwater supplies silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. Here, we examine the benthic cycling of the two nutrients at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including Si and Fe isotopes (δ³⁰Si and δ⁵⁶Fe), and reaction‐transport modeling for Si. A high diffusive flux of dissolved Fe from the fjord sediments (up to 0.02 mmol m⁻² day⁻¹) compared to open ocean sediments (typically 0.1 mmol m⁻² day⁻¹). High pore water δ³⁰Si (up to +3.3‰) observed near the Fe(II)‐Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe, yet suppresses the release of dissolved Si at glaciated fjord heads, which has significant implications for understanding the downstream transport of these nutrients along fjord systems.

The flux of terrestrial material from the continents to the oceans links the lithosphere, hydrosphere, and biosphere through physical and biogeochemical processes, with important implications for Earth's climate. Quantitative estimates of terrigenous fluxes from sources such as rivers, aeolian dust, and resuspended shelf sediments are required to understand how the processes delivering terrigenous material respond to and are influenced by climate. We compile thorium‐230 normalized²³²Th flux records in the tropical Atlantic to provide an improved understanding of aeolian fluxes since the Last Glacial Maximum (LGM). By identifying and isolating sites dominated by aeolian terrigenous inputs, we show that there was a persistent meridional gradient in dust fluxes in the eastern equatorial Atlantic at the LGM, arguing against a large southward shift of the intertropical convergence zone during LGM boreal winter. The ratio of LGM to late‐Holocene²³²Th fluxes highlights a meridional difference in the magnitude of variations in dust deposition, with sites 700 km away, characterized by²³²Th fluxes approximately twice as large as aeolian‐dominated sites in the east.

Absolutely dated deep-sea coral ∆ ¹⁴ C, δ ¹¹ B, and δ ¹⁵ N records reveal Southern Ocean’s role in rapid deglacial carbon cycle events.

Biogeochemical cycling of silicon (Si) in the high latitudes has an important influence on the marine Si budget. The Barents Sea is divided aproximately equally into Arctic and Atlantic water (ArW and AW respectively) domains.  However, increases in the temperature and inflow of AW across the Barents Sea opening is driving an expansion of the AW realm. While the sensitivity of pelagic processes pertaining to primary production is receiving increasingly more attention, less is known of the effect on the benthic Si cycle. This knowledge gap could prove integral, as the flux of Si across the sediment-water interface (SWI) from Arctic shelf sediments could be up to 20% higher than that of riverine sources. This benthic flux is largely controlled by early diagenetic processes in sediment pore waters, including biogenic silica (bSi) dissolution and authigenic precipitation.To improve our understanding of benthic Si dynamics in the Barents Sea and examine its sensitivity to future change, we analysed pore water and sediment samples from both the AW and ArW realms between 2017-2019 for dissolved silica (dSi) concentrations and stable silicon isotopic compositions. Moreover, to determine the composition and content of bSi, as well as Si sorbed onto metal oxides, we conducted a sequential digestion of surface sediment. Following this we coupled our analyses with reaction transport modelling to further improve our mechanistic understanding of the system and to quantitatively disentangle the relative importance of these diagenetic processes to pore water Si chemistry and benthic fluxes.Our work suggests that both interannual and spatial variability of dSi are increased in the southern, AW region of the Barents Sea. Benthic flux estimates for the southern sites have been found to more than double (~30 to 100 mmol m-2 yr-2) between cruise years, compared to a more consistent flux in the north (~80 mmol m-2 yr-2). Therefore, future Atlantification of the northern region may enance the variability of dSi supply from the benthos to bottom waters, with potential consequences for diatom productivity in the region.