title: Dating marine shell in Oceania: Issues and prospects

authors: Petchey, Fiona

subjects: marine reservoir

abstract: Marine shell has several advantages for radiocarbon (¹⁴C) dating in the Pacific — it is ubiquitous in archaeological sites, is easy to identify to the species level, and can often be related directly to human activity. Consequently, shells are one of the most commonly dated ¹⁴C sample types within this region. The modelled marine calibration curve and associated regional offsets (known as ΔR) originally constructed by Stuiver et al. (1986), have been widely accepted as the most accurate method for calibrating surface marine ¹⁴C dates. The use of published values, however, is not straightforward because the surface ocean ¹⁴C reservoir is variable both regionally and over time, and because of additional uncertainties with the reliability of some shell species due to habitat and dietary preferences. This paper presents an overview of ΔR variability in Oceania and highlights areas of caution when using extant ΔR values, and when selecting marine shell for ¹⁴C dating. Particular attention is given to the Hawaiian archipelago where numerous ΔR values are available for evaluation and the influence of ocean currents, estuarine environments and geology is apparent.

text: Dating marine shell in Oceania: Issues and prospects Marine shell has several advantages for radiocarbon (¹⁴C) dating in the Pacific — it is ubiquitous in archaeological sites, is easy to identify to the species level, and can often be related directly to human activity. Consequently, shells are one of the most commonly dated ¹⁴C sample types within this region. The modelled marine calibration curve and associated regional offsets (known as ΔR) originally constructed by Stuiver et al. (1986), have been widely accepted as the most accurate method for calibrating surface marine ¹⁴C dates. The use of published values, however, is not straightforward because the surface ocean ¹⁴C reservoir is variable both regionally and over time, and because of additional uncertainties with the reliability of some shell species due to habitat and dietary preferences. This paper presents an overview of ΔR variability in Oceania and highlights areas of caution when using extant ΔR values, and when selecting marine shell for ¹⁴C dating. Particular attention is given to the Hawaiian archipelago where numerous ΔR values are available for evaluation and the influence of ocean currents, estuarine environments and geology is apparent.

year: 2009

title: Emerging stable isotope techniques in hydrothermal research: new methods applied to carbonates and hydrous minerals in ore deposits and geothermal fields

authors: Mering, John

subjects: stable isotope

abstract: Over the last 70 years, stable isotopes have provided critical insight into the processes governing transfer of mass and energy through geologic systems. Significantly, isotopic signals in minerals reflect conditions of crystallisation, and subsequent alteration history. In hydrothermal systems, isotope tools have been applied to interpret fluid origin, temperature, and interaction with host rock. The research carried out here makes use of emerging techniques to measure hydrogen, oxygen, carbon, and clumped isotopes in carbonates and hydrous minerals. This work encompasses three themes: 

(i)	Development of new laser spectroscopy approaches for measuring δ2H and δ18O in hydrous minerals 
(ii)	Testing carbonate clumped isotope thermometry in geothermal fields and mineral deposits 
(iii)	Application of carbonate and hydrous mineral isotope tools in epithermal precious metal deposits 

A novel approach to measure δ2H and δ18O in hydrous minerals is presented in Chapters 3-4. Measurement was carried out on phyllosilicates, hydrous sulphates, and multimineralic whole rock assay pulps using a custom-built thermal dehydration-dehydroxylation Off-Axis Integrated Cavity Output Spectroscopy (TD-OA-ICOS) system. The δ2H methodology presented is equal to isotope ratio mass spectrometry in terms of speed and precision, but considerably lower in cost. This work represents the first hydrogen isotope measurements made by a laser spectroscopy system for serpentine, muscovite, sericite, talc, and biotite. 

The TD-OA-ICOS method was subsequently applied to measure δ18O in microvolumes of water, gypsum hydration water, and the hydroxyl component of phyllosilicate minerals (Chapter 4). Intramineral oxygen isotope fractionation values in kaolinite and muscovite, determined by comparison of δ18OOH measured by TD-OA-ICOS with δ18Osilicate measured by fluorination, are similar to existing constraints. Refinement of this approach should be carried out to test the relationship between formation temperature and intramineral oxygen isotope fractionation in clay minerals. 
TD-OA-ICOS was applied to reconstruct δ2H in well-constrained epithermal areas, exemplified by case studies at the Comstock Lode, USA (Chapter 4) and Miocene-aged adularia-sericite Au-Ag deposits at Waihi and Karangahake on the North Island (Chapter 6). Taking mineral-specific fractionation factors into account, whole rock hydrogen isotope measurements indicate relatively homogeneous fluids of meteoric origin were involved in alteration at each deposit. Accurate reconstruction of fluid δ2H requires constraints upon the balance of illite and chlorite in samples. Mineral δ2H is useful for fingerprinting paleo latitude or altitude of source fluids, as evidenced by results from Comstock, which were ~60 ‰ lower for δ2H, relative to Waihi and Karangahake. It is apparent that mineral δ2H has greater utility towards understanding mixing in systems where fluids are isotopically distinct. The low cost, safety, and simplicity of operation of TD-OA-ICOS should also make the method attractive for manufacturers and regulatory agencies wishing to use hydrogen and oxygen isotopes to determine the geographic origin of consumer products containing hydrous minerals.

Paleotemperature constraints around ore deposits are essential for characterising the relationship between advection, fluid circulation, and metal transport. In Chapter 5, carbonate clumped isotope thermometry (Δ47) is applied to identify temperature gradients in geothermal fields and hydrothermal ore deposits. This technique, based on quantification of thermodynamically sensitive 13C-18O bond abundance in carbonates, provides new capability to determine fluid δ18O, overcoming a problem which has undermined interpretation of carbonate isotope data in hydrothermal settings for more than 60 years. Geothermal fields (Wairakei, Ngatamariki, Broadlands-Ohaaki) in the Taupo Volcanic Zone, New Zealand served as natural laboratories to test the validity of clumped isotope calibrations in high temperature settings up to 300 °C. Taking local fluid δ18O into account, results indicate modern TVZ calcite precipitated in equilibrium with produced geothermal waters.

The clumped isotope technique was subsequently applied in epithermal (Waihi, NZ), skarn (Antamina, Peru), and carbonate-hosted (Mount Isa, Australia) metal deposits. Results highlight the potential for clumped isotopes to delineate the heat footprint around mineral deposits that contain carbonates. Further refinement of the clumped isotope method offers new potential to evaluate characteristics of ore deposit genesis, with respect to temperature, fluid source, and depth. This will prove especially useful in enigmatic carbonate-hosted hydrothermal settings, where temperature and hydrology are unresolved. Paired oxygen isotope and carbonate clumped isotope measurements at Waihi demonstrate that δ18O of hydrothermal calcite is primarily influenced by rock temperature, with variation in fluid composition due to water-rock exchange exerting secondary control. While calcite precipitates both in and out of phase with precious metals in epithermal areas (e.g. Waihi), clumped isotope signals reflect the overall thermal structure, providing a new avenue to identify hotter upwelling zones, where boiling drives deposition of Au, Ag and other metals. Ultimately, the findings presented herein are useful to workers in hydrothermal settings, with interest in mineral exploration and geothermal resource development, and are broadly applicable across the geologic sciences in topics ranging from paleoenvironmental studies to interpretation of basin histories.

text: Emerging stable isotope techniques in hydrothermal research: new methods applied to carbonates and hydrous minerals in ore deposits and geothermal fields Over the last 70 years, stable isotopes have provided critical insight into the processes governing transfer of mass and energy through geologic systems. Significantly, isotopic signals in minerals reflect conditions of crystallisation, and subsequent alteration history. In hydrothermal systems, isotope tools have been applied to interpret fluid origin, temperature, and interaction with host rock. The research carried out here makes use of emerging techniques to measure hydrogen, oxygen, carbon, and clumped isotopes in carbonates and hydrous minerals. This work encompasses three themes: 

(i)	Development of new laser spectroscopy approaches for measuring δ2H and δ18O in hydrous minerals 
(ii)	Testing carbonate clumped isotope thermometry in geothermal fields and mineral deposits 
(iii)	Application of carbonate and hydrous mineral isotope tools in epithermal precious metal deposits 

A novel approach to measure δ2H and δ18O in hydrous minerals is presented in Chapters 3-4. Measurement was carried out on phyllosilicates, hydrous sulphates, and multimineralic whole rock assay pulps using a custom-built thermal dehydration-dehydroxylation Off-Axis Integrated Cavity Output Spectroscopy (TD-OA-ICOS) system. The δ2H methodology presented is equal to isotope ratio mass spectrometry in terms of speed and precision, but considerably lower in cost. This work represents the first hydrogen isotope measurements made by a laser spectroscopy system for serpentine, muscovite, sericite, talc, and biotite. 

The TD-OA-ICOS method was subsequently applied to measure δ18O in microvolumes of water, gypsum hydration water, and the hydroxyl component of phyllosilicate minerals (Chapter 4). Intramineral oxygen isotope fractionation values in kaolinite and muscovite, determined by comparison of δ18OOH measured by TD-OA-ICOS with δ18Osilicate measured by fluorination, are similar to existing constraints. Refinement of this approach should be carried out to test the relationship between formation temperature and intramineral oxygen isotope fractionation in clay minerals. 
TD-OA-ICOS was applied to reconstruct δ2H in well-constrained epithermal areas, exemplified by case studies at the Comstock Lode, USA (Chapter 4) and Miocene-aged adularia-sericite Au-Ag deposits at Waihi and Karangahake on the North Island (Chapter 6). Taking mineral-specific fractionation factors into account, whole rock hydrogen isotope measurements indicate relatively homogeneous fluids of meteoric origin were involved in alteration at each deposit. Accurate reconstruction of fluid δ2H requires constraints upon the balance of illite and chlorite in samples. Mineral δ2H is useful for fingerprinting paleo latitude or altitude of source fluids, as evidenced by results from Comstock, which were ~60 ‰ lower for δ2H, relative to Waihi and Karangahake. It is apparent that mineral δ2H has greater utility towards understanding mixing in systems where fluids are isotopically distinct. The low cost, safety, and simplicity of operation of TD-OA-ICOS should also make the method attractive for manufacturers and regulatory agencies wishing to use hydrogen and oxygen isotopes to determine the geographic origin of consumer products containing hydrous minerals.

Paleotemperature constraints around ore deposits are essential for characterising the relationship between advection, fluid circulation, and metal transport. In Chapter 5, carbonate clumped isotope thermometry (Δ47) is applied to identify temperature gradients in geothermal fields and hydrothermal ore deposits. This technique, based on quantification of thermodynamically sensitive 13C-18O bond abundance in carbonates, provides new capability to determine fluid δ18O, overcoming a problem which has undermined interpretation of carbonate isotope data in hydrothermal settings for more than 60 years. Geothermal fields (Wairakei, Ngatamariki, Broadlands-Ohaaki) in the Taupo Volcanic Zone, New Zealand served as natural laboratories to test the validity of clumped isotope calibrations in high temperature settings up to 300 °C. Taking local fluid δ18O into account, results indicate modern TVZ calcite precipitated in equilibrium with produced geothermal waters.

The clumped isotope technique was subsequently applied in epithermal (Waihi, NZ), skarn (Antamina, Peru), and carbonate-hosted (Mount Isa, Australia) metal deposits. Results highlight the potential for clumped isotopes to delineate the heat footprint around mineral deposits that contain carbonates. Further refinement of the clumped isotope method offers new potential to evaluate characteristics of ore deposit genesis, with respect to temperature, fluid source, and depth. This will prove especially useful in enigmatic carbonate-hosted hydrothermal settings, where temperature and hydrology are unresolved. Paired oxygen isotope and carbonate clumped isotope measurements at Waihi demonstrate that δ18O of hydrothermal calcite is primarily influenced by rock temperature, with variation in fluid composition due to water-rock exchange exerting secondary control. While calcite precipitates both in and out of phase with precious metals in epithermal areas (e.g. Waihi), clumped isotope signals reflect the overall thermal structure, providing a new avenue to identify hotter upwelling zones, where boiling drives deposition of Au, Ag and other metals. Ultimately, the findings presented herein are useful to workers in hydrothermal settings, with interest in mineral exploration and geothermal resource development, and are broadly applicable across the geologic sciences in topics ranging from paleoenvironmental studies to interpretation of basin histories.

year: 2019

title: Effects of lake trophic state and seasonal cycling on production and bioaccumulation of methylmercury in Te Arawa Lakes

authors: Chong, Grace-Lynn

abstract: There are natural sources of Mercury in New Zealand, primarily derived from volcanic activity and geothermal vents. Mercury (Hg) and its organic derivative, methylmercury (MeHg), are highly toxic to both the environment and human health. Of particular concern is the ability of MeHg to bioaccumulate in aquatic food webs, leading to increased toxicity in higher trophic organisms. Given that fish constitute a significant component of the local diet, comprehensive monitoring of Hg and MeHg concentrations is essential for developing effective mitigation strategies. Such efforts are critical to controlling Hg contamination in the environment and aquatic biota, as well as informing the public about lakes with elevated mercury levels. Previous Hg studies in New Zealand have been limited in scope, with monitoring efforts focused on only a few lakes rather than a comprehensive assessment across multiple ecosystems. Additionally, most existing research has primarily examined elemental Hg, with limited investigation into the distribution and bioavailability of MeHg.   

This study aims to comprehensively assess Hg and MeHg concentrations in 11 lakes of the Taupo Volcanic Zone. The study assessed Hg and MeHg concentrations in lake water and aquatic biota, including trout, crayfish (koura), and mussels (kakahi). To achieve this, Hg and MeHg in the water were measured using diffusive gradients in thin films (DGT) samplers, while tissue analyses determined Hg, MeHg and additional trace elements in the selected species. The findings of this study are expected to clarify the distribution and bioaccumulation of Hg and MeHg within lake ecosystems and to elucidate their transfer through the aquatic food web. These results will provide a scientific basis for assessing ecological and human health risks associated with mercury exposure, particularly for culturally and recreationally important species. The outcomes will inform lake management and monitoring strategies by identifying key pathways of mercury uptake and potential bioindicator species, and may guide the development of targeted mitigation measures, such as catchment management or advisories on species consumption. Collectively, this research will support evidence-based decision-making for freshwater ecosystem protection and long-term contaminant management.  

The results of this study revealed that total mercury (THg) and MeHg concentrations in trout, koura, and kakahi from Lakes Rotorua, Rotoiti, and Rotomahana exceeded WHO recommended safety thresholds. This study also found that a substantial proportion of THg was comprised of MeHg, which was predominantly accumulated in muscle tissue, whereas THg was primarily concentrated in the liver. The findings further indicated that eutrophication, lake stratification and hypolimnetic anoxia contribute to the enhanced production and bioaccumulation of Hg and MeHg within these aquatic systems. Additionally, selenium (Se) demonstrated a potential protective effect against MeHg toxicity in aquatic organisms.

text: Effects of lake trophic state and seasonal cycling on production and bioaccumulation of methylmercury in Te Arawa Lakes There are natural sources of Mercury in New Zealand, primarily derived from volcanic activity and geothermal vents. Mercury (Hg) and its organic derivative, methylmercury (MeHg), are highly toxic to both the environment and human health. Of particular concern is the ability of MeHg to bioaccumulate in aquatic food webs, leading to increased toxicity in higher trophic organisms. Given that fish constitute a significant component of the local diet, comprehensive monitoring of Hg and MeHg concentrations is essential for developing effective mitigation strategies. Such efforts are critical to controlling Hg contamination in the environment and aquatic biota, as well as informing the public about lakes with elevated mercury levels. Previous Hg studies in New Zealand have been limited in scope, with monitoring efforts focused on only a few lakes rather than a comprehensive assessment across multiple ecosystems. Additionally, most existing research has primarily examined elemental Hg, with limited investigation into the distribution and bioavailability of MeHg.   

This study aims to comprehensively assess Hg and MeHg concentrations in 11 lakes of the Taupo Volcanic Zone. The study assessed Hg and MeHg concentrations in lake water and aquatic biota, including trout, crayfish (koura), and mussels (kakahi). To achieve this, Hg and MeHg in the water were measured using diffusive gradients in thin films (DGT) samplers, while tissue analyses determined Hg, MeHg and additional trace elements in the selected species. The findings of this study are expected to clarify the distribution and bioaccumulation of Hg and MeHg within lake ecosystems and to elucidate their transfer through the aquatic food web. These results will provide a scientific basis for assessing ecological and human health risks associated with mercury exposure, particularly for culturally and recreationally important species. The outcomes will inform lake management and monitoring strategies by identifying key pathways of mercury uptake and potential bioindicator species, and may guide the development of targeted mitigation measures, such as catchment management or advisories on species consumption. Collectively, this research will support evidence-based decision-making for freshwater ecosystem protection and long-term contaminant management.  

The results of this study revealed that total mercury (THg) and MeHg concentrations in trout, koura, and kakahi from Lakes Rotorua, Rotoiti, and Rotomahana exceeded WHO recommended safety thresholds. This study also found that a substantial proportion of THg was comprised of MeHg, which was predominantly accumulated in muscle tissue, whereas THg was primarily concentrated in the liver. The findings further indicated that eutrophication, lake stratification and hypolimnetic anoxia contribute to the enhanced production and bioaccumulation of Hg and MeHg within these aquatic systems. Additionally, selenium (Se) demonstrated a potential protective effect against MeHg toxicity in aquatic organisms.

year: 2025