The marine carbon cycle is vitally important for climate and the fertility of the oceans. However, predictions of future biogeochemistry are challenging because a myriad of processes need ...parameterization and the future evolution of the physical ocean state is uncertain. Here, we embed a data-constrained model of the carbon cycle in slower and warmer ocean states as simulated under the RCP4.5 and RCP8.5 (RCP: Representative Concentration Pathway) scenarios for the 2090s and frozen in time for perpetuity. Focusing on steady-state changes from preindustrial conditions allows us to capture the response of the system integrated over all the timescales of the steady-state biogeochemistry, as opposed to typical transient simulations that capture only sub-centennial timescales. We find that biological production experiences only modest declines (of 8 %–12 %) because the reduced nutrient supply due to a more sluggish circulation and strongly shoaled mixed layers is counteracted by warming-stimulated growth. Organic-matter export declines by 15 %–25 % due to reductions in both biological production and export ratios, the latter being driven by warming-accelerated shallow respiration and reduced subduction of dissolved organic matter. The perpetual-2090s biological pump cycles a 30 %–70 % larger regenerated inventory accumulated over longer sequestration times, while preformed DIC is shunted away from biological utilization to outgassing. The regenerated and preformed DIC inventories both increase by a similar magnitude. We develop a conceptually new partitioning of preformed DIC to quantify the ocean's preformed carbon pump and its changes. Near-surface paths of preformed DIC are more important in the slower circulations, as weakened ventilation isolates the deep ocean. Thus, while regenerated DIC cycling becomes slower, preformed DIC cycling speeds up.
Abstract
We constrain tropospheric transport from Northern Hemisphere midlatitudes to the Southern Hemisphere (SH) surface using measurements of SF
6
, CFCs, and CFC replacement gases and a novel ...maximum‐entropy‐based inversion approach. We provide the first estimate of the width Δ of the tropospheric interhemispheric transit time distribution (TTD). We find that Δ has a value of ∼1.3 years that varies little with SH latitude, compared to the mean transit time Γ that increases from ∼1.1 years in the SH tropics to ∼1.4 years at the South Pole. The TTD shape parameter Δ/Γ is thus larger in the SH tropics than at middle and high SH latitudes. Our analysis introduces a simple path‐dependent lifetime that parameterizes chemical losses. The path‐dependent lifetimes are estimated for CFC replacements, and systematic differences between path‐dependent and global lifetimes are interpreted. The path‐dependent lifetimes have the potential to provide new observational constraints on tropospheric and stratospheric loss processes.
Key Points
NH‐to‐SH mean transit time increases from 1.1 years at Samoa to 1.4 years at the pole
The width of the TTD varies little with SH latitude and is about 1.3 years
Path‐dependent and global lifetimes differ based on reactivity and transit time
We constrain tropospheric transport from Northern Hemisphere midlatitudes to the Southern Hemisphere (SH) surface using measurements of SF sub(6), CFCs, and CFC replacement gases and a novel ...maximum-entropy-based inversion approach. We provide the first estimate of the width Delta of the tropospheric interhemispheric transit time distribution (TTD). We find that Delta has a value of 1.3years that varies little with SH latitude, compared to the mean transit time Gamma that increases from 1.1years in the SH tropics to 1.4years at the South Pole. The TTD shape parameter Delta / Gamma is thus larger in the SH tropics than at middle and high SH latitudes. Our analysis introduces a simple path-dependent lifetime that parameterizes chemical losses. The path-dependent lifetimes are estimated for CFC replacements, and systematic differences between path-dependent and global lifetimes are interpreted. The path-dependent lifetimes have the potential to provide new observational constraints on tropospheric and stratospheric loss processes. Key Points * NH-to-SH mean transit time increases from 1.1years at Samoa to 1.4years at the pole * The width of the TTD varies little with SH latitude and is about 1.3years * Path-dependent and global lifetimes differ based on reactivity and transit time
Abstract
The intimate relationship among ventilation, transit-time distributions, and transient tracer budgets is analyzed. To characterize the advective–diffusive transport from the mixed layer to ...the interior ocean in terms of flux we employ a cumulative ventilation-rate distribution, Φ(τ), defined as the one-way mass flux of water that resides at least time τ in the interior before returning. A one-way (or gross) flux contrasts with the net advective flux, often called the subduction rate, which does not accommodate the effects of mixing, and it contrasts with the formation rate, which depends only on the net effects of advection and diffusive mixing. As τ decreases Φ(τ) increases, encompassing progressively more one-way flux. In general, Φ is a rapidly varying function of τ (it diverges at small τ), and there is no single residence time at which Φ can be evaluated to fully summarize the advective–diffusive flux. To reconcile discrepancies between estimates of formation rates in a recent GCM study, Φ(τ) is used. Then chlorofluorocarbon data are used to bound Φ(τ) for Subtropical Mode Water and Labrador Sea Water in the North Atlantic Ocean. The authors show that the neglect of diffusive mixing leads to spurious behavior, such as apparent time dependence in the formation, even when transport is steady.
Abstract
Radiocarbon (Δ
14
C) and helium isotopes (
δ
3
He) have long been used to constrain the ocean's ventilation rates and to trace regional deep ocean circulation pathways, but they have not ...been fully exploited together to constrain the deep circulation in global models. Here we assimilate Δ
14
C and
δ
3
He measurements into a global ocean circulation inverse model (OCIM) to jointly constrain the deep ocean circulation and the rate of mantle‐helium injection at seafloor spreading ridges. We find that the new version of the inverse model (OCIM2) matches the observed Δ
14
C and
δ
3
He distributions much better than a previous version (OCIM1) that assimilated objectively mapped Δ
14
C but not
δ
3
He. OCIM2 features faster‐ventilated bottom waters and slower‐ventilated intermediate‐depth waters in the Pacific and Indian Oceans. The mean time since
last
ventilation (ideal mean age) in Pacific bottom waters is up to 150 years younger, while middepth Pacific waters are up to several hundred years older. The
δ
3
He constraints are shown to be important for estimates of the mean time to
next
ventilation in the Pacific Ocean. The
δ
3
He constraints also favor jet‐like currents in the deep equatorial Pacific to capture realistic westward propagating helium plumes emanating from the East Pacific Rise. The globally integrated mantle‐helium source is 585–672 mol/year, compared to 400–1,000 mol/year from previous estimates. The major regional difference occurs in the Southern Ocean, where the OCIM2 mantle‐helium source is up to threefold smaller than estimates based on ridge spreading rates.
Key Points
Radiocarbon and helium isotope measurements provide complementary constraints on the deep ocean ventilation in a circulation inverse model
The assimilated circulation produces realistic westward propagating
3
He plumes in the deep Eastern Tropical Pacific
The jointly assimilated globally integrated mantle‐
3
He source is 585–672 mol/year
We show that the flux of mass crossing in one direction (the “gross” flux) through any specified surface S that divides anadvective‐diffusive flow in a closed domain is infinite. That is, the flux, ...(τ), through S of the fluid mass that spent at least time τ on one side of S diverges like τ−1/2 as τ → 0, in the continuum limit. The gross flux is completely dominated by fluid elements residing infinitesimally short times on one side of S before re‐crossing to the other side. This general result puts into context the widely varying estimates of gross mass flux across the midlatitude tropopause. Such estimates are dominated by the smallest resolved scales, leading us to conclude that gross mass flux is not a useful diagnostic of stratosphere‐troposphere exchange. The function (τ), however, provides important information on transport across the tropopause.
The global distributions of the silicon isotopes within silicic acid are estimated by adding isotope fractionation to an optimized, data‐constrained model of the oceanic silicon cycle that is ...embedded in a data‐assimilated steady circulation. Including fractionation during opal dissolution improves the model's ability to capture the approximately linear relation between isotope ratio, δ30Si, and inverse silicic acid concentration observed in the deep Atlantic. To quantify the importance of hydrographic control on the isotope distribution, δ30Si is partitioned into contributions from preformed and regenerated silicic acid, further partitioned according to euphotic zone origin. We find that the large‐scale features of the isotope distribution in the Atlantic basin are dominated by preformed silicic acid, with regenerated silicic acid being important for setting vertical gradients. In the Pacific and Indian Oceans, preformed and regenerated silicic acid make roughly equally important contributions to the pattern of the isotope ratio, with gradients of the preformed and regenerated contributions tending to cancel each other in the deep Pacific. The Southern Ocean euphotic zone is the primary origin of both the preformed and regenerated contributions to δ30Si. Nearly the entire preformed part of δ30Si is of Southern Ocean and North Atlantic origin. The regenerated part of δ30Si in the Atlantic basin also has a contribution of Central Atlantic (∼40°S–40°N) origin that is comparable in magnitude to the North Atlantic contribution. In other basins, the Central Pacific and Indian Ocean are the second largest contributors to the regenerated part of δ30Si.
Key Points
Both preformed and regenerated silicic acid shape the Si isotope distribution
More than two end‐members contribute to the Atlantic Si isotope distribution
Fractionation with opal dissolution leads to a better fit of the observations
The ocean's nutrient cycles are important for the carbon balance of the climate system and for shaping the ocean's distribution of dissolved elements. Dissolved iron (dFe) is a key limiting ...micronutrient, but iron scavenging is observationally poorly constrained, leading to large uncertainties in the external sources of iron and hence in the state of the marine iron cycle. Here we build a steady-state model of the ocean's coupled phosphorus, silicon, and iron cycles embedded in a data-assimilated steady-state global ocean circulation. The model includes the redissolution of scavenged iron, parameterization of subgrid topography, and small, large, and diatom phytoplankton functional classes. Phytoplankton concentrations are implicitly represented in the parameterization of biological nutrient utilization through an equilibrium logistic model. Our formulation thus has only three coupled nutrient tracers, the three-dimensional distributions of which are found using a Newton solver. The very efficient numerics allow us to use the model in inverse mode to objectively constrain many biogeochemical parameters by minimizing the mismatch between modeled and observed nutrient and phytoplankton concentrations. Iron source and sink parameters cannot jointly be optimized because of local compensation between regeneration, recycling, and scavenging. We therefore consider a family of possible state estimates corresponding to a wide range of external iron source strengths. All state estimates have a similar mismatch with the observed nutrient concentrations and very similar large-scale dFe distributions. However, the relative contributions of aeolian, sedimentary, and hydrothermal iron to the total dFe concentration differ widely depending on the sources. Both the magnitude and pattern of the phosphorus and opal exports are well constrained, with global values of 8. 1 ± 0. 3 Tmol P yr−1 (or, in carbon units, 10. 3 ± 0. 4 Pg C yr−1) and 171. ± 3. Tmol Si yr−1. We diagnose the phosphorus and opal exports supported by aeolian, sedimentary, and hydrothermal iron. The geographic patterns of the export supported by each iron type are well constrained across the family of state estimates. Sedimentary-iron-supported export is important in shelf and large-scale upwelling regions, while hydrothermal iron contributes to export mostly in the Southern Ocean. The fraction of the global export supported by a given iron type varies systematically with its fractional contribution to the total iron source. Aeolian iron is most efficient in supporting export in the sense that its fractional contribution to export exceeds its fractional contribution to the total source. Per source-injected molecule, aeolian iron supports 3. 1 ± 0. 8 times more phosphorus export and 2. 0 ± 0. 5 times more opal export than the other iron types. Conversely, per injected molecule, sedimentary and hydrothermal iron support 2. 3 ± 0. 6 and 4. ± 2. times less phosphorus export, and 1. 9 ± 0. 5 and 2. ± 1. times less opal export than the other iron types.
Respiration Patterns in the Dark Ocean Sulpis, Olivier; Trossman, David S.; Holzer, Mark ...
Global biogeochemical cycles,
August 2023, 2023-08-00, 20230801, Volume:
37, Issue:
8
Journal Article
Peer reviewed
Open access
In the dark ocean, respiring organisms are the main sink for dissolved oxygen. The respiration rate in a given seawater volume can be quantified through dissolved oxygen drawdown or organic matter ...consumption as a function of time. Estimates of dissolved oxygen utilization rates (OUR) abound in the literature, but are typically obtained using proxies of questionable accuracy, often with low vertical resolution, and neglecting key regions such as the Southern and Indian oceans. Respiration rates based on particulate (POC) or dissolved (DOC) organic carbon are also sparsely observed and for DOC are unavailable in many regions. Consequently, the relative contributions of POC or DOC as a respiration substrate in the dark ocean are unknown. Here, we use recent datasets of true oxygen utilization, seawater age, and DOC to derive OUR and DOC consumption‐rate profiles in 10 oceanic regions. We demonstrate that although DOC and POC consumption rates are globally consistent with OUR, they underestimate OUR in the deep, suggesting strong oxygen utilization at the seafloor. In the abyss, we find a negative correlation of the DOC consumption rate with seawater age, suggesting that DOC reactivity decreases along the deep branch of the conveyor circulation. Our results highlight that benthic organisms are sensitive to perturbations in the surface production of organic matter and to large‐scale circulation changes that affect its supply to the abyss.
Key Points
DOC is important for microbial respiration in the abyssal ocean where the DOC consumption rate decreases with seawater mean age
About 8% of O2 utilization in the midnight zone and in the abyssal ocean is attributed to processes occurring at the seafloor
Total dark ocean O2 consumption (907 Tmol O2 a−1) is balanced by sediment O2 (74 Tmol O2 a−1) and organic C consumption (727 Tmol C a−1)
Here we present a new flexible modeling tool for simulating the distribution of tracers in the modern ocean. A Working Environment for Simulating Ocean Movement and Elemental cycling within an Ocean ...Circulation Inverse Model, the AWESOME OCIM, is a transport matrix model (TMM) which is specifically designed to be easy, accessible, and intuitive, even for scientists without prior modeling experience. The AWESOME OCIM comes with a variety of selectable biogeochemical functions, including sources (atmospheric dust, hydrothermal vents, and seafloor nepheloid layers), internal cycling processes (biological uptake, remineralization, and scavenging), and sinks (radioactive decay and burial of particles in the sediments). A wide variety of elements can be simulated through different combinations of this suite of processes. We anticipate that the AWESOME OCIM will be a valuable tool for interpreting transect data from ocean surveys, particularly the trace-elements and isotopes distributions mapped by the ongoing GEOTRACES program. This manuscript provides an introduction to the philosophical, mathematical, and functional basis of the AWESOME OCIM.
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