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'Seawater' has been substantially updated in this second edition to take account of recent developments in marine science.Sections dealing with difficult physical and chemical concepts have been developed on the basis of feedback from the first edition, making this an ideal learning tool for oceanography students.Chapter 1 summarizes the special.

Quantification of palaeo pH and palaeo CO2 from marine proxies requires the use of apparent equilibrium constants (K∗’s). The K∗’s required to calculate seawater carbonate chemistry are described by empirically determined calibrations, and for palaeoseawater these calibrations must be adjusted to account for changes in seawater composition. There are a number of possible calibrations that can be used to calculate K∗’s, and each software package for calculating carbonate chemistry separately implements these K∗ calibrations, which are manually verified against previous implementations. Palaeo applications typically only implement a single calibration for each K∗, but have additional complexity from the need to adjust each calibration for changing seawater composition through time. The fragmented approach to K∗ calculation can result in unintended and difficult to resolve discrepancies between outputs calculated by different pieces of software. We present “Kgen,” a synchronised collection of three pieces of software to consistently calculate palaeoseawater K∗’s in Python, R, and Matlab. Kgen provides a simple, and nearly identical interface in each language. Through use of a Continuous Integration/Continuous Delivery (CI/CD) pipeline, Kgen guarantees consistent outputs between languages by automatically cross‐checking results from all three implementations. Unifying the approach to K∗ calculation in this way provides an extensible platform for verifiable K∗ generation for palaeoseawater, which can be integrated into existing carbon speciation calculators to improve the consistency of results. Plain Language Summary To calculate how various compounds in seawater behave (chemically speaking), it is necessary to calculate what are known as “apparent equilibrium constants” (denoted by the symbol K∗). These K∗’s are used to determine proportions of compounds which are present in seawater in a variety of forms. K∗’s are also sensitive to the chemical make‐up of seawater, which has changed through time. Before the work presented here, K∗ calculation was usually done as a necessary aside within other software. This is an issue, because different pieces of software can calculate them in subtly different ways (in particular the adjustment for changing chemical composition), which leads to differences in the outputs of these packages which are difficult to resolve. In this work, we present a software package called Kgen that is exclusively devoted to calculating K∗’s. The software is co‐written in three programming languages that are routinely used for seawater calculations, and we integrate an automatic testing procedure to guarantee that all three languages always produce the same results. Kgen provides a more coherent approach to K∗ generation that can be integrated into existing software. Key Points Kgen is a software package to calculate apparent equilibrium constants for palaeoseawater carbonate system calculations Kgen accounts for the influences of temperature, pressure, salinity, magnesium concentration, and calcium concentration We provide a new implementation of the MyAMI model (“pymyami”) to adjust apparent equilibrium constants for changing seawater composition

Understanding the impacts that mineral grain size and seawater salinity have on magnesium hydroxide (Mg(OH)2) dissolution and secondary calcium carbonate (CaCO3) precipitation is critical for the success of ocean alkalinity enhancement. We tested Mg(OH)2 dissolution kinetics in seawater using three Mg(OH)2 grain sizes (<63, 63–180 and >180 µm) at three salinities (∼36, ∼28 and ∼20). While Mg(OH)2 dissolution occurred more quickly the smaller the grain size, salinity did not significantly impact measured rates. Our results also demonstrate that grain size can impact secondary CaCO3 precipitation, suggesting that an optimum grain size exists for ocean alkalinity enhancement (OAE) using solid Mg(OH)2. Of the three grain sizes tested, the medium grain size (63–180 µm) was optimal in terms of delaying secondary CaCO3 precipitation. We hypothesise that in the lowest-grain-size experiments, the higher surface area provided numerous CaCO3 precipitation nuclei, while the slower dissolution of bigger grain sizes maintained a higher alkalinity and pH at the surface of particles, increasing CaCO3 precipitation rates and making them observable much more quickly than for the intermediate grain size. Salinity also played a role in CaCO3 precipitation, where the decrease in magnesium (Mg) allowed secondary precipitation to occur more quickly, similar in effect size to another known inhibitor, i.e. dissolved organic carbon (DOC). In summary, our results suggest that OAE efficiency as influenced by CaCO3 precipitation depends not only on seawater composition but also on the physical properties of the alkaline feedstock used.

The neodymium isotope signatures (εNd) of the authigenic fraction have been extensively used to reconstruct past seawater εNd and hydrological circulation. Among the various methods, sequential extraction of hydrogenic ferromanganese oxyhydroxides from bulk sediments represents a rapid and straightforward approach that may potentially induce artifacts due to the potential release of non‐seawater‐derived Nd during the extraction procedure. Here we investigated different methods for extracting past seawater Nd isotope compositions from a core collected in the Adriatic Sea whose tephra layers have been previously well documented. We analyzed εNd in planktonic foraminifera samples and in non‐decarbonated sediment leachates obtained with three solutions commonly used in the context of the Mediterranean Sea: (a) 0.02 M hydroxylamine hydrochloride (HH) solution, (b) 1N HCl, and (b) a 25% (v/v) acetic acid (AA). Our results show that (a) the foraminiferal εNd remains unaffected by the diagenesis of tephra content; (b) all three methods indicate significantly more radiogenic εNd values in tephra levels (up to 1.5 εNd unit), which is attributed to tephra dissolution accounting for 2.7% of extracted Nd; (c) of the three leaching methods applied to samples with low tephra content, hydroxylamine hydrochloride (HH) yields εNd values that are more consistent with those obtained on planktonic foraminifera; (d) the εNd values of planktonic foraminifera in core MD90‐917 remained constant indicating that the Adriatic deep water primarily reflects the local Nd isotope composition over the last 20 kyr. Plain Language Summary Neodymium isotope signatures (εNd) from the authigenic fraction of sediments are commonly used to reconstruct past ocean circulation and seawater composition. A common method involves extracting ferromanganese oxyhydroxides from bulk sediments, which is quick and straightforward but may introduce contamination from non‐seawater sources of neodymium during the process. In this study, we tested different methods for extracting εNd from a sediment core in the Adriatic Sea, known for its tephra layers, by comparing data from foraminifera and sediment leachates using three solutions: hydroxylamine hydrochloride (HH), HCl, and acetic acid (AA). We found that: (a) foraminiferal εNd is unaffected by tephra dissolution; (b) all methods showed more radiogenic εNd in tephra layers due to tephra dissolution; (c) the HH solution gave εNd values closest to foraminifera in low‐tephra samples; (d) foraminiferal εNd has remained stable over the last 20,000 years, indicating Adriatic deep water reflects local neodymium isotope composition. Key Points The HCl and AA leaching processes dissolve the tephras and bias the extracted authigenic εNd values εNd of HH leachate closely matches that of foraminifera and past seawater of the Adriatic Sea εNd of the Adriatic Deep Water stay stable during the last 20,000 years due to vertical water mixing and local lithogenic inputs

The Cretaceous/Paleogene (K/Pg) Chicxulub impact produced globally synchronous enrichments of highly siderophile elements (HSEs) and a pronounced decline in marine osmium isotope ratios (187Os/188Os), forming a key geochemical marker of the boundary. However, precise identification of the K/Pg boundary remains challenging in extensive pelagic regions dominated by deep‐sea red clay, where calcareous and/or siliceous microfossils are poorly preserved. Here we identify the K/Pg boundary in a pelagic clay sequence from piston core KR13‐02 PC05 recovered near Minamitorishima Island in the western Pacific Ocean using HSE concentrations and Os isotope stratigraphy, supported by ichthyolith biostratigraphy and magnetostratigraphy. A distinct Ir enrichment (up to ∼16 ppb) coincides with the most unradiogenic 187Os/188Os value (∼0.14) at ∼9.95 mbsf, defining a geochemical horizon consistent with the global signature of the Chicxulub impact. Polarity chrons spanning C29n–C30n and ichthyolith assemblages support the placement of the K/Pg boundary and provide a robust chronological framework. Above the boundary, 187Os/188Os ratios remain persistently unradiogenic over a stratigraphic interval corresponding to several hundred thousand years. This prolonged signal reflects the integrated effects of extremely low sedimentation rates, sediment reworking by bioturbation, and background extraterrestrial dust inputs. These results demonstrate that sedimentary Os isotope records in low‐accumulation pelagic clay do not necessarily represent a simple archive of contemporaneous seawater composition. Instead, depositional and diagenetic processes can substantially modify post‐impact isotopic signals. Our findings highlight the necessity of multi‐proxy chronological control and careful geochemical screening when reconstructing oceanic responses to the K/Pg impact on microfossil‐poor pelagic environments.

This study presents high-precision analyses of stable potassium (K) isotope ratio using the recently-developed, collision-cell multi-collector inductively coupled plasma mass spectrometry (CC-MC-ICP-MS, Nu Sapphire). The accuracy of our analyses is confirmed by measuring well-characterized geostandards (including rocks and seawater). Our results are consistent with literature values and a precision of 0.04‰ (2SD) has been achieved based on multiple measurements of BCR-2 geostandard over a six-month period. We also evaluate factors that may lead to artificial isotope fractionations, including the mismatches in K concentration and acid molarity between samples and bracketing standards, as well as potential matrices. As the K adsorption capacity of AGW50-X8 (200–400 mesh) is reduced with an increasing amount of matrix elements, less than 150 µg K was loaded during the column chemistry. To evaluate the potential use of K isotopes as an archive of paleo seawater composition, δ 41 K values of an international seawater standard (IAPSO), a Mn-nodule (NOD-P-1), and two iron formation standards (FeR-2 and FeR-4) are reported. The δ 41 K value of IAPSO is consistent with other seawater samples reported previously, further substantiating a homogeneous K isotopic distribution in modern global oceans. The K isotopes in Mn-nodule (NOD-P-1: −0.121±0.013‰) and iron formation samples (FeR-2: −0.538±0.009‰; FeR-4: −0.401±0.008‰) seem to be an effective tracer of their formation genesis and compositional changes of ancient seawater. Our results suggest that high-precision measurements of stable K isotopes can be routinely obtained and open up a large variety of geological applications, such as continental weathering, hydrothermal circulation and alteration of oceanic crust.

Fetida Cave is an active sulfuric acid cave influenced by seawater, showing abundant microbial communities that organize themselves under three main different morphologies: water filaments, vermiculations and moonmilk deposits. These biofilms/deposits have different cave distribution, pH, macro- and microelement and mineralogical composition, carbon and nitrogen content. In particular, water filaments and vermiculations had circumneutral and slightly acidic pH, respectively, both had abundant organic carbon and high microbial diversity. They were rich in macro- and microelements, deriving from mineral dissolution, and, in the case of water filaments, from seawater composition. Vermiculations had different color, partly associated with their mineralogy, and unusual minerals probably due to trapping capacities. Moonmilk was composed of gypsum, poor in organic matter, had an extremely low pH (0-1) and low microbial diversity. Based on 16S rRNA gene analysis, the microbial composition of the biofilms/deposits included autotrophic taxa associated with sulfur and nitrogen cycles and biomineralization processes. In particular, water filaments communities were characterized by bacterial taxa involved in sulfur oxidation and reduction in aquatic, aphotic, microaerophilic/anoxic environments (Campylobacterales, Thiotrichales, Arenicellales, Desulfobacterales, Desulforomonadales) and in chemolithotrophy in marine habitats (Oceanospirillales, Chromatiales). Their biodiversity was linked to the morphology of the water filaments and their collection site. Microbial communities within vermiculations were partly related to their color and showed high abundance of unclassified Betaproteobacteria and sulfur-oxidizing Hydrogenophilales (including Sulfuriferula), and Acidiferrobacterales (including Sulfurifustis), sulfur-reducing Desulfurellales, and ammonia-oxidizing Planctomycetes and Nitrospirae. The microbial community associated with gypsum moonmilk showed the strong dominance (>60%) of the archaeal genus Thermoplasma and lower abundance of chemolithotrophic Acidithiobacillus, metal-oxidizing Metallibacterium, Sulfobacillus, and Acidibacillus. This study describes the geomicrobiology of water filaments, vermiculations and gypsum moonmilk from Fetida Cave, providing insights into the microbial taxa that characterize each morphology and contribute to biogeochemical cycles and speleogenesis of this peculiar seawater-influenced sulfuric acid cave.

Bromine radicals influence global tropospheric chemistry by depleting ozone and by oxidizing elemental mercury and reduced sulfur species. Observations typically indicate a 50 % depletion of sea salt aerosol (SSA) bromide relative to seawater composition, implying that SSA debromination could be the dominant global source of tropospheric bromine. However, it has been difficult to reconcile this large source with the relatively low bromine monoxide (BrO) mixing ratios observed in the marine boundary layer (MBL). Here we present a new mechanistic description of SSA debromination in the GEOS-Chem global atmospheric chemistry model with a detailed representation of halogen (Cl, Br, and I) chemistry. We show that observed levels of SSA debromination can be reproduced in a manner consistent with observed BrO mixing ratios. Bromine radical sinks from the HOBr + S(IV) heterogeneous reactions and from ocean emission of acetaldehyde are critical in moderating tropospheric BrO levels. The resulting HBr is rapidly taken up by SSA and also deposited. Observations of SSA debromination at southern midlatitudes in summer suggest that model uptake of HBr by SSA may be too fast. The model provides a successful simulation of free-tropospheric BrO in the tropics and midlatitudes in summer, where the bromine radical sink from the HOBr + S(IV) reactions is compensated for by more efficient HOBr-driven recycling in clouds compared to previous GEOS-Chem versions. Simulated BrO in the MBL is generally much higher in winter than in summer due to a combination of greater SSA emission and slower conversion of bromine radicals to HBr. An outstanding issue in the model is the overestimate of free-tropospheric BrO in extratropical winter–spring, possibly reflecting an overestimate of the HOBr∕HBr ratio under these conditions where the dominant HOBr source is hydrolysis of BrNO3.

Reconstruction of the evolutionary history of Earth’s habitability has been approached with various geochemical proxies of sedimentary rocks, particularly carbonates. It is a consensus that marine carbonates recorded the seawater geochemical compositions unless the primary signals were altered in diagenesis. This argument is the cornerstone for the applications of carbonate-based geochemical proxies on chemostratigraphic correlations and paleoenvironment interpretations. However, this assumption is only valid for carbonate precipitation in the water column, whilst most carbonate rocks outcropped in continents were precipitated on the seafloor. The modern seafloor commonly displays upward diffusions of various reduced components, e.g., Fe 2+ , Mn 2+ , H 2 S, and dissolved inorganic carbon (DIC), from sediment porewater to seawater, collectively called the benthic flux. As such, carbonates precipitated on the seafloor are inevitably affected by benthic flux from porewater, recording mixed signals of both seawater and porewater. In this study, we first reviewed the precipitation modes of marine carbonate. Then we explore how carbonate geochemical compositions could be affected by benthic flux when precipitated on the seafloor, and suggest using ferrous Fe content in carbonate (Fe carb ) as a proxy to constrain benthic flux. Finally, with the consideration of benthic flux, we evaluate the traditional interpretation of carbonate carbon isotope and sulfur isotope of carbonate associated sulfate (CAS). We show that carbonate geochemical compositions do not reflect the seawater composition unless the benthic flux is negligible. Nevertheless, the seawater geochemical compositions, which could be used for regional or global chemostratigraphic correlations, could be retrieved by excluding benthic flux signals, whereas the extracted benthic flux signals could be used to confine the seafloor redox conditions. We suggest that interpretations of carbonate geochemical data should base on rigorous petrographic and sedimentological analyses.

Phosphorites of the Georgina Basin (northern Australia) are an established economic source of phosphate and have recently been recognized to be a potential source of rare earth elements (REE). Previous bulk‐rock geochemistry work focused on the eastern margin of the basin revealed that phosphorites from the southern region have significantly higher (up to 0.5%) REE contents than equivalent prospects further to the north. In this study, we examine the origin of REE enrichment in the Georgina Basin phosphorites using an integrated geochemical, petrographic, mineralogical, and isotopic (Sm‐Nd and Sr) approach. The trace element geochemistry of primary phosphate minerals is consistent with a seawater origin for the REEs in phosphorites, with minimal input from the underlying basement rocks. Variations in total REE concentrations are controlled largely by the seawater composition, depositional environment, and the texture of the phosphorite (grainstone vs. mudstone phosphorite). Grainstone phosphorite yields on average higher concentrations of REEs compared to the mudstone phosphorite, likely due to the higher surface‐to‐mass ratio of the grainstone phosphorite resulting in the uptake of higher concentrations of REEs during coastal reworking. The formation of the low‐REE northern and central deposits took place during early transgression in oxidized, shallow supratidal to subtidal platformal environments. During the subsequent main transgressive event, fault‐controlled topography in the southern domain allowed phosphogenesis along basement highs to be facilitated along a redox boundary, where more saline and REE‐rich deep anoxic bottom waters, along with Fe‐cycling and bacterial mediation played a major role in the enrichment of REEs in the southern phosphorites. Key Points The origin of rare earth element (REE) enrichment in phosphorites of the Georgina Basin in northern Australia, was determined using an integrated geochemical, mineralogical, and isotopic approach The enrichment of REEs in phosphorites is controlled by the texture and the depositional environment at the time of phosphorite formation Varying levels of REE enrichment occurred at different stages of marine transgression