Accurate radiocarbon dating of marine samples requires knowledge of the marine radiocarbon reservoir effect. This effect for a particular site/region is generally assumed constant through time when ...calibrating marine 14C ages. However, recent studies have shown large temporal variations of several hundred to a couple of thousand years in this effect for a number of regions during the late Quaternary and Holocene. Here we report marine radiocarbon reservoir correction (ΔR) for Heron Reef and Moreton Bay in southwestern (SW) Pacific for the last 8 ka derived from 14C analysis of 230Th-dated corals. Most of our ΔR for the last ∼5.4 ka agree well with their modern value, but large ΔR variability of ∼410 yr (from trough to peak) with possible decadal/centennial fluctuations is evident for the period ∼5.4–8 ka. The latter time interval also has significant variations with similar features in previously published ΔR values for other sites in the Pacific, including southern Peru–northern Chile in southeastern (SE) Pacific, the South China Sea, Vanuatu and Papua New Guinea, with the largest magnitude of ∼920 yr from SE Pacific. The mechanisms for these large ΔR variations across the Pacific during the mid-Holocene are complex processes involving (1) changes in the quantity and 14C content of upwelled waters in tropical east Pacific (TEP) (frequency and intensity of ocean upwelling in the TEP, and contribution of Subantarctic Mode Water to the upwelled waters, which is influenced by the intensity and position of southern westerly winds), and (2) variations in ocean circulation associated with climate change (La Niña/El Niño conditions, intensity of easterly trade winds, positions of the Intertropical Convergence Zone and the South Pacific Convergence Zone), which control the spreading of the older upwelled surface waters in the TEP to the western sites. Our results imply the need for employing temporal changes in ΔR values, instead of constant (modern) values, for age calibration of Holocene marine samples not only for the SW Pacific sites but also for other tropical and subtropical sites in the Pacific.
•New ΔR values for SW Pacific for 0–8 ka based on 14C analysis of 230Th-dated corals.•Large ΔR variability across the Pacific, including the SW sites, for ∼5.4–8 ka.•Synchronous ΔR variations indicate large scale climate and ocean circulation drivers.•Urgent need for using temporal ΔR changes for improved 14C dating of marine samples.
Evidence from physical and geochemical tracers measured during the World Ocean Circulation Experiment (WOCE) shows that there are four sub-types of Antarctic Intermediate Water (AAIW) in the South ...Pacific. The main formation region of AAIW is the southeast Pacific, where fresh, cold, high oxygen, low nutrient, intermediate waters are created. This AAIW is transported north and mixes with Equatorial Pacific Intermediate Waters (EqPIW), themselves a combination of AAIW and nutrient rich, old North Pacific deep waters. ‘Tasman’ AAIW found in the Coral and Tasman Seas is more saline and warmer than the main subtropical gyre, and appears to have formed from mixing of AAIW with thermocline waters in the Tasman Gyre. Tasman AAIW leaks out of the Tasman basin to the north of New Zealand and along Chatham Rise, and also in the South Tasman Sea via the Tasman Leakage. Another source of relatively fresh, high oxygen, low nutrient, young AAIW comes directly from the Southern Ocean, flowing into the southwest and central South Pacific Basin, west of the East Pacific Rise. This ‘Southern Ocean’ (SO) AAIW is most likely a mixture of AAIW formed locally at the Subantarctic Front (SAF), and AAIW formed along the SAF in the southeast Pacific or Indian oceans and transported by the Antarctic Circumpolar Current (ACC). Interpreting physical and geochemical tracers, combined with velocity estimates from Argo floats, and previous research, has allowed us to refine the detailed circulation pattern of AAIW in the South Pacific, especially in the topographically complex southwest Pacific.
► 4 types of AAIW are identified in the South Pacific from their geochemical characteristics. ► AAIW circulation in the South Pacific is revised from the geochemistry, Argo trajectories and previous studies. ► Improved understanding of the mixing and circulation of AAIW in the topographically complex SW Pacific.
A coral reef represents the net accumulation of calcium carbonate (CaCO$_3$) produced by corals and other calcifying organisms. If calcification declines, then reef-building capacity also declines. ...Coral reef calcification depends on the saturation state of the carbonate mineral aragonite of surface waters. By the middle of the next century, an increased concentration of carbon dioxide will decrease the aragonite saturation state in the tropics by 30 percent and biogenic aragonite precipitation by 14 to 30 percent. Coral reefs are particularly threatened, because reef-building organisms secrete metastable forms of CaCO$_3$, but the biogeochemical consequences on other calcifying marine ecosystems may be equally severe.
Human-induced ocean acidification and warming alter seawater carbonate chemistry reducing the calcification of reef-building crustose coralline algae (CCA), which has implications for reef stability. ...However, due to the presence of multiple carbonate minerals with different solubilities in seawater, the algal mineralogical responses to changes in carbonate chemistry are poorly understood. Here we demonstrate a 200% increase in dolomite concentration in living CCA under greenhouse conditions of high pCO2 (1,225 μatm) and warming (30 °C). Aragonite, in contrast, increases with lower pCO2 (296 μatm) and low temperature (28 °C). Mineral changes in the surface pigmented skeleton are minor and dolomite and aragonite formation largely occurs in the white crust beneath. Dissolution of high-Mg-calcite and particularly the erosive activities of endolithic algae living inside skeletons play key roles in concentrating dolomite in greenhouse treatments. As oceans acidify and warm in the future, the relative abundance of dolomite in CCA will increase.
In tropical and sub‐tropical mixed siliciclastic–carbonate depositional systems, fluvial input and in situ neritic carbonate interact over space and time. Despite being the subject of many studies, ...controls on partitioning of mixed sediments remains controversial. Mixed sedimentary records, from Ashmore Trough shelf edge and slopes (southern Gulf of Papua), are coupled with global sea‐level curves and anchored to Marine Isotope Stage stratigraphy to constrain models of sediment accumulation at two different timescales for the past 130 kyr: (i) 100 kyr scale for last glacial cycle; and (ii) millennial scale for last deglaciation. During the last glacial cycle, carbonate production and accumulation were primarily controlled by sea‐level fluctuations. Export of neritic carbonate to the slopes was initiated during re‐flooding of previously exposed reefs and continued during Marine Isotope Stage 5e and 1 interglacial sea‐level highs. Siliciclastic fluxes to the slope were controlled by interplay of sea level, shelf physiography and oceanic currents. Heterogeneous accumulation of siliciclastic mud on the slope, took place during Marine Isotope Stage 5d to Marine Isotope Stage 3 sea‐level fall. Siliciclastics reached adjacent depocentres during Marine Isotope Stage 2. Coralgal reef and oolitic–skeletal sand resumed at the shelf edge during the subsequent stepwise sea‐level rise of the last deglaciation. Contemporaneous, abrupt siliciclastic input from increased precipitation and fluvial discharge illustrates that climate controlled deglacial sedimentation. Siliciclastic input persisted until ca 8.5 ka. Carbonate accumulation waned at the shelf edge after ca 14 ka, whereas it increased on the slopes since ca 11.5 ka, when previously exposed reef and bank tops were re‐flooded. When comparing the last sea‐level cycle sedimentation patterns of the southern Gulf of Papua with other coeval mixed systems, sea level and shelf physiography emerge as primary controls on deposition at the 100 kyr scale. At the millennial scale, siliciclastic input was also controlled by climate change during the unstable atmospheric and oceanic conditions of the last deglaciation.
It is predicted that surface ocean pH will reach 7.9, possibly 7.8 by the end of this century due to increased carbon dioxide (CO
2) in the atmosphere and in the surface ocean. While aragonite-rich ...sediments don't begin to dissolve until a threshold pH of ~
7.8 is reached, dissolution from high-Mg calcites is evident with any drop in pH. Indeed, it is high-Mg calcite that dominates the reaction of carbonate sediments with increased CO
2, which undergoes a rapid neomorphism process to a more stable, low-Mg calcite. This has major implications for the future of the high-Mg calcite producing organisms within coral reef ecosystems. In order to understand any potential buffering system offered by the dissolution of carbonate sediments under a lower oceanic pH, this process of high-Mg calcite dissolution in the reef environment must be further elucidated.
► With increased atmospheric CO
2, ocean pH is predicted to drop significantly. ► We find that a decrease in ocean pH will lead to dissolution of carbonate reef sediments. ► Aragonite and low-Mg calcite sediments will react when pH reaches 7.8. ► High-Mg calcite reacts with any drop in pH, with reprecipitation of low-Mg calcite.
The oceans are becoming more acidic due to absorption of anthropogenic carbon dioxide from the atmosphere. The impact of ocean acidification on marine ecosystems is unclear, but it will likely depend ...on species adaptability and the rate of change of seawater pH relative to its natural variability. To constrain the natural variability in reef-water pH, we measured boron isotopic compositions in a approximately300-year-old massive Porites coral from the southwestern Pacific. Large variations in pH are found over approximately50-year cycles that covary with the Interdecadal Pacific Oscillation of ocean-atmosphere anomalies, suggesting that natural pH cycles can modulate the impact of ocean acidification on coral reef ecosystems.
The [simple carbon project] model v1.0 O'Neill, Cameron M.; Hogg, Andrew McC; Ellwood, Michael J. ...
Geoscientific Model Development,
04/2019, Letnik:
12, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We construct a carbon cycle box model to process observed or inferred geochemical evidence from modern and paleo settings. The simple carbon project model v1.0 (SCP-M) combines a modern understanding ...of the ocean circulation regime with the Earth's carbon cycle. SCP-M estimates the concentrations of a range of elements within the carbon cycle by simulating ocean circulation, biological, chemical, atmospheric and terrestrial carbon cycle processes. The model is capable of reproducing both paleo and modern observations and aligns with CMIP5 model projections. SCP-M's fast run time, simplified layout and matrix structure render it a flexible and easy-to-use tool for paleo and modern carbon cycle simulations. The ease of data integration also enables model–data optimisations. Limitations of the model include the prescription of many fluxes and an ocean-basin-averaged topology, which may not be applicable to more detailed simulations. In this paper we demonstrate SCP-M's application primarily with an analysis of the carbon cycle transition from the Last Glacial Maximum (LGM) to the Holocene and also with the modern carbon cycle under the influence of anthropogenic CO2 emissions. We conduct an atmospheric and ocean multi-proxy model–data parameter optimisation for the LGM and late Holocene periods using the growing pool of published paleo atmosphere and ocean data for CO2, δ13C, Δ14C and the carbonate ion proxy. The results provide strong evidence for an ocean-wide physical mechanism to deliver the LGM-to-Holocene carbon cycle transition. Alongside ancillary changes in ocean temperature, volume, salinity, sea-ice cover and atmospheric radiocarbon production rate, changes in global overturning circulation and, to a lesser extent, Atlantic meridional overturning circulation can drive the observed LGM and late Holocene signals in atmospheric CO2, δ13C, Δ14C, and the oceanic distribution of δ13C, Δ14C and the carbonate ion proxy. Further work is needed on the analysis and processing of ocean proxy data to improve confidence in these modelling results.
This paper outlines the evolution of the late Cenozoic mixed carbonate‐siliciclastic depositional system in the Gulf of Papua (GoP), using seismic, gravity, multibeam bathymetry, well data sets, and ...Landsat imagery. The deposition of the mixed sedimentary sequences was influenced by dynamic interplay of tectonics, eustasy, in situ carbonate production, and siliciclastic sediment supply. The roles of these major factors are estimated during different periods of the GoP margin evolution. The Cenozoic mixed system in the GoP formed in distinct phases. The first phase (Late Cretaceous–Paleocene) was mostly driven by tectonics. Rifting created grabens and uplifted structural blocks which served later as pedestals for carbonate edifices. Active neritic carbonate accumulation characterized the second phase (Eocene–middle Miocene). During this phase, mostly eustatic fluctuations controlled the large‐scale sedimentary geometries of the carbonate system. The third phase (late Miocene–early Pliocene) was characterized by extensive demise of the carbonate platforms in the central part of the study area, which can be triggered by one or combination of several factors, such as eustatic sea level fluctuations, increased tectonic subsidence, uplift, sudden influx of siliciclastics, or dramatic changes in environmental conditions and climate. The fourth phase (late Pliocene‐Holocene) was dominated by siliciclastics, which resulted in the burial of drowned and/or active carbonate platforms, although some platforms still remain alive until present‐day.
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