Difference In δ18O Research Articles

Calcification depth of deep-dwelling planktonic foraminifera from the eastern North Atlantic constrained by stable oxygen isotope ratios of shells from stratified plankton tows

Abstract. Stable oxygen isotopes (δ18O) of planktonic foraminifera are one of the most used tools to reconstruct environmental conditions of the water column. Since different species live and calcify at different depths in the water column, the δ18O of sedimentary foraminifera reflects to a large degree the vertical habitat and interspecies δ18O differences and can thus potentially provide information on the vertical structure of the water column. However, to fully unlock the potential of foraminifera as recorders of past surface water properties, it is necessary to understand how and under what conditions the environmental signal is incorporated into the calcite shells of individual species. Deep-dwelling species play a particularly important role in this context since their calcification depth reaches below the surface mixed layer. Here we report δ18O measurements made on four deep-dwelling Globorotalia species collected with stratified plankton tows in the eastern North Atlantic. Size and crust effects on the δ18O signal were evaluated showing that a larger size increases the δ18O of G. inflata and G. hirsuta, and a crust effect is reflected in a higher δ18O signal in G. truncatulinoides. The great majority of the δ18O values can be explained without invoking disequilibrium calcification. When interpreted in this way the data imply depth-integrated calcification with progressive addition of calcite with depth to about 300 m for G. inflata and to about 500 m for G. hirsuta. In G. scitula, despite a strong subsurface maximum in abundance, the vertical δ18O profile is flat and appears dominated by a surface layer signal. In G. truncatulinoides, the δ18O profile follows equilibrium for each depth, implying a constant habitat during growth at each depth layer. The δ18O values are more consistent with the predictions of the Shackleton (1974) palaeotemperature equation, except in G. scitula which shows values more consistent with the Kim and O'Neil (1997) prediction. In all cases, we observe a difference between the level where most of the specimens were present and the depth where most of their shell appears to calcify.

Palaeodiet inferred from pre‐Hispanic and early colonial human remains from Carrizales, Zaña Valley, Peru

AbstractThis study utilises multi‐isotopic analysis to reconstruct diet and source‐water consumption from human remains collected at Carrizales, in the Zaña Valley of northern coastal Peru. Carrizales is a multicomponent site that encompasses the remains of (a) Early Intermediate Period (200–800 CE) cemeteries (Conjuntos 126 and 127), (b) Late Sicán/Lambayeque period (ca. 1100–1350 CE) domestic occupations (Conjuntos 125, 128, and 131), and (c) a planned town (reducción) into which indigenous people were resettled under Spanish rule, ca. 1572 CE (Conjunto 123). Faunal and botanical assemblages at the site differ significantly between the Late Sicán/Lambayeque and early colonial phases, suggesting a distinct shift in local subsistence regimes following Spanish colonisation and resettlement.Human remains were collected from Carrizales for the characterisation of dietary isotopes and further examination of the impacts of Spanish colonialism on native foodways. Carbon, nitrogen, and oxygen isotope values were characterised in the enamel carbonate, bone carbonate, and bone collagen of individuals associated with Early Intermediate Period contexts (N = 4) and the late 16th century chapel within the site's reducción (N = 21). Over half of the study sample failed to produce viable bone collagen, an issue that is common in bioarchaeological samples from the north coast of Peru. However, among those with viable bone collagen and carbonate, there were minimal differences in δ13C, δ18O, and δ15N values between the two phases. This extends to values in enamel carbonate, which is highly resistant to diagenesis. Therefore, patterns in isotope values do not mirror the significant differences manifest in food remains from Carrizales. We suggest that (a) the diets of Early Intermediate Period individuals and Early Colonial period individuals buried at the site were quite similar, contrasting with the patterns indicated by Late Intermediate Period food remains, and (b) both populations obtained water from similar sources—a pattern that reinforces the characterisation of reducción in the Zaña Valley as a relatively local phenomenon in which nearby populations were concentrated into planned towns, rather than drawn from distant valleys.

Stable Oxygen Isotope Records (δ18O) of a High-Andean Cushion Peatland in NW Argentina (24° S) Imply South American Summer Monsoon Related Moisture Changes During the Late Holocene

High-elevation cushion peatlands are promising archives for paleoenvironmental studies in their extreme habitat of the Central Andean highlands between ~4000 and 4800m a.s.l. The Cerro Tuzgle cushion peatland (CTP, 24°09’ S, 66°24’ W), located in the NW Argentine Andes, is formed by the vascular cushion plants Oxychloe andina (O. andina) and Zameioscirpus muticus (Z. muticus). To extend the knowledge base on the modern ecology of these peatlands, we investigated the stable isotope composition of bulk material and cellulose (δ18O, δ13C, δ15N) of the dominant cushion-forming species O. andina (Juncaceae) and Z. muticus (Cyperaceae) as well as water samples (δ18O, δ2H) of several pools interspersed within the peatland. We further applied a multiproxy approach for a peat core from CTP spanning the last 2900 years with XRF scanning, bulk geochemistry and stable isotope analyses on bulk peat and cellulose size fractions. Modern samples of O. andina and Z. muticus expose significant differences in cellulose δ18O e.g. between leaves and rhizomes of O. andina (Δδ18Ol-r = 4.11‰) and between leaves of O. andina and Z. muticus (Δδ18Ol-l = 2.8‰). Modern water samples exhibit strong isotopic differences between single water pools (max. Δδ18O = 13.09‰) due to local variable evaporative enrichment. Within the peat core, we observe considerable multi-centennial variations in δ18O composition of cellulose confirmed by all size fractions. Based on the regional relation between decreasing δ18Oprec values with increasing precipitation amounts and 18O enrichment in the peatland waters due to evaporation, we suggest an interpretation of our δ18O cellulose record as moisture proxy for CTP. This interpretation is corroborated by a high correlation between oxygen isotopes, peat growth and geochemical data. Accordingly, CTP indicates dryer conditions between 2190 and 2120, 1750 and 1590, 1200 and 1080 and since 130 cal. yr BP, whereas periods with increased humidity prevailed from 2750 to 2250 and from 600 to 130 cal. yr BP. Temporal changes in the match to South American Summer Monsoon (SASM) reconstructions suggest impacts of other large-scale atmospheric variability modes or a different SASM expression at our southerly location.

A new speleothem record of the penultimate deglacial: Insights into spatial variability and centennial-scale instabilities of East Asian monsoon

We present a highly resolved oxygen isotope (δ18O) record of the penultimate glacial-interglacial transition from a 230Th-dated stalagmite (SD1) from the Shangxiaofeng Cave in north-eastern China. Our δ18O record reveals two prominent features. First, the SD1 δ18O record documents a brief increase of δ18O between 128.5 and 128.1 ka BP. Though this event has been identified in several marine records, our radiogenic dated record precisely constraints the timing and duration of the climate deterioration suggesting that this YD-like event is likely an intrinsic feature of climate change during the ice age terminations. Additionally, the δ18O magnitude of the glacial-interglacial change (∼0.7‰) is significantly smaller compared to the δ18O records from inland and relatively high elevation caves (Dongge, Sanbao and Yangkou caves) (1.4–2.4‰) but comparable to the Hulu Cave at low elevation and proximal to the shallow Bohai and Yellow seas (Kelly et al., 2006; Cheng et al., 2006; Wang et al., 2008; Cheng et al., 2009; Li et al., 2014). Assuming that the cave δ18O records mainly reflect precipitation isotope composition changes, the glacial-interglacial speleothem δ18O difference between the various Chinese caves could be related to changes in atmospheric circulations, changing effect of altitude between the various caves and climate mean states, and large-scale changes in regional land-sea coverage due to sea level changes. The latter leads to disappearance and expansion of Bohai and Yellow seas during glacial and interglacial episodes, respectively, and likely affect the amount and isotope composition of precipitation. Taken together, our study highlights that hydroclimate imprints of the East Asian Monsoon changes during the penultimate glacial-interglacial cycle were spatially heterogeneous and centennial-scale monsoonal hydroclimate instabilities are likely inherent features of deglacial climate transition.

An Assessment of Plant Species Differences on Cellulose Oxygen Isotopes From Two Kenai Peninsula, Alaska Peatlands: Implications for Hydroclimatic Reconstructions

Peat cores are valuable archives of past environmental change because they accumulate plant organic matter over millennia. While studies have primarily focused on physical, ecological, and some biogeochemical proxies, cores from peatlands have increasingly been used to interpret hydroclimatic change using stable isotope analyses of cellulose preserved in plant remains. Previous studies indicate that the stable oxygen isotope compositions (δ18O) preserved in alpha cellulose extracted from specific plant macrofossils reflect the δ18O values of past peatland water and thereby provide information on long-term changes in hydrology in response to climate. Oxygen isotope analyses of peat cellulose (δ18Ocellulose) have been successfully developed from peat cores that accumulate the same species for millennia. However, to fully exploit the potential of this proxy in species-diverse fens, studies are needed that account for the isotopic variations caused by changes in dominant species composition. This study assesses variation in δ18O values among peatland plant species and how they relate to environmental waters in two fens informally named Horse Trail and Goldfin, located on the leeward (dry) and windward (wet) side, respectively, of the climatic gradient across the Kenai Peninsula, Alaska. Environmental water δ18O values at both fens reflect unmodified δ18O values of mean annual precipitation, although at Goldfin standing pools were slightly influenced by evaporation. Modern plant (mosses and Carex (sedges)) δ18Ocellulose values indicate that all Carex are higher (~2.5 ‰) than those of mosses, likely driven by their vascular structure and ecophysiological difference from non-vascular mosses. Moss δ18Ocellulose values within each peatland are similar among the species, and differences appear related to evaporation effects on environmental waters within hummocks and hollows. The plant taxa-environmental water δ18O differences are applied to the previously determined Horse Trail Fen untreated bulk δ18O record. Results include significant changes to inferred millennial-to-centennial scale hydroclimatic trends where dominant taxa shift from moss to Carex, indicating that modern calibration datasets are necessary for interpreting stable isotopes from fens, containing a mix of vascular and nonvascular plants. Accounting for isotopic offsets through macrofossil analysis and modern plant-water isotope measurements opens new opportunities for hydroclimatic reconstructions from fen peatlands.

Is Chinese stalagmite δ18O solely controlled by the Indian summer monsoon?

As a unique continental archive, speleothem has been widely used in reconstructing paleoclimate change. However, the interpretation of Chinese speleothems δ18O has remained a subject of debate. Recently, a Community Atmosphere Model version 3 (CAM3) study indicated that the stalagmite δ18O from eastern China reflected the Indian summer monsoon (ISM) intensity rather than the East Asian summer monsoon (EASM) intensity during Heinrich events. Here, we present a high-resolution speleothem δ18O record from Xianglong Cave in Shaanxi province, China, covering the period of 25.5–10.9 ka BP. The XL15 record shows similar variations with ice core record from Greenland and other climate records from China and India on millennial scale, including Heinrich 2 (H2), Heinrich 1 (H1), Bolling–Allerod (BA) and Younger Dryas (YD) events, supporting the connection between the Asian monsoon and northern high latitude climate. The δ18O amplitude of our record is larger than or similar to the stalagmite δ18O records from India during these events. In addition, differences of stalagmite δ18O in eastern China and the ISM region were observed on glacial-interglacial as well as decadal timescales. That means the ISM is not the sole controlling factor of Chinese stalagmite δ18O during Heinrich events. When subtracting the Indian stalagmite δ18O series from our XL15 record during H1 period, we found a significant negative correlation with sea surface temperature (SST) record of Western Pacific Warm Pool (WPWP). Consequently, our study suggests that the Chinese stalagmite δ18O is controlled by both the ISM and EASM on orbital-, millennial-, and decadal timescales.

Cumulative effects of cascade dams on river water cycle: Evidence from hydrogen and oxygen isotopes

Cascade dams are known to influence the river water cycle, but their cumulative effects (CEs) are still not well understood. Water hydrogen (H) and oxygen (O) isotopes are hypothesized to be used to characterize the CEs. To test this hypothesis, we investigated water δD, δ18O, and related environmental factors in cascade reservoirs on the Wujiang River, Southwest China. The δD and δ18O ranged from −64.2‰ to −45.4‰ and from −9.7‰ to −6.8‰, respectively, and showed obvious temporal and spatial variations. Water temperature is an important factor influencing these variations. After damming, an increase of water retention time caused the enrichment of heavy H-O isotopes in reservoir surface water, and thermal stratification induced a decrease of δD and δ18O with depth. Due to bottom discharge, released water showed more negative δD and δ18O than reservoir surface water, and these δD and δ18O differences were controlled by water retention time and mean water depth of the reservoir. Overall, the CEs of cascade dams caused δD and δ18O to display a jagged increase from upstream to downstream in the impounded Wujiang River. Therefore H-O isotopes can be used to estimate the CEs of cascade dams. As cascade dams can modify H-O isotope signatures, caution should be exercised when using H and O isotopes to trace the source of the impounded river water.

Stable isotopes in water indicate sources of nutrients that drive algal blooms in the tributary bay of a subtropical reservoir

Eutrophication has become a severe environmental problem in some tributaries of the Three Gorges Reservoir (TGR) in China. A two-year field investigation of nutrients, oxygen stable isotopes (δ18O), and hydrogen stable isotopes (δD) was performed from January 2010 to December 2011 to determine the sources of nutrients in Xiangxi Bay (XXB). The results showed that nitrogen, phosphorus and silicon varied seasonally depending on hydrodynamic changes. The bottom-layer intrusive density current decreased nitrogen and silicon concentrations and increase phosphorus concentrations in XXB, while the middle-layer intrusive density current increased nitrogen and silicon concentrations and decrease phosphorus concentrations. The differences in δ18O and δD among the Yangtze River (YR), XXB and the region upstream of XXB were significant, and according to the tracer method, the estimated contribution ratios of nitrogen, phosphorus and silicon from the YR to XXB were much larger than those from the region upstream of XXB. These findings suggest that water quality in the TGR can be improved by reducing the pollution load throughout the upstream basin of the YR but not through decentralized efforts in only one or two tributary basins.

Monsoon variability and upper ocean stratification during the last ∼66 ka over the Andaman Sea: Inferences from the δ18O records of planktonic foraminifera

Using new oxygen isotope (δ18O) data of three planktonic foraminiferal (Globigerinoides ruber, Globigerinoides sacculifer and Globorotalia menardii) species from a sediment core SK-234-60, collected from the Andaman Sea, we infer that summer monsoon was weaker during Marine Isotopic Stage-3 (MIS-3) than the early Holocene but was stronger than that during MIS-2. Further, an abrupt decrease in a monsoon during the mid-Holocene between ∼7 and 4.5 ka is observed. The difference in δ18O between species with different habitats depth also enables the reconstruction of past ocean stratification over the Andaman Sea. Distinct weak and strong freshwater-induced upper ocean stratifications are observed during the last ∼66 ka. A weaker upper ocean stratification during the Last Glacial Maximum (LGM) and a stronger stratification during the early Holocene is observed. This is due to the prevailing positive and negative Evaporation minus Precipitation (E-P) conditions, respectively. Weaker stratifications during ∼32 ka and early to mid-MIS-3 are observed, whereas the stratification was strong during ∼42-39 ka. Stronger monsoon associated with higher upper ocean stratification was seen during ∼62 ka. Likewise, weaker monsoon during the Heinrich events was associated with weaker stratification.

Assessing the seasonality and uncertainty in evapotranspiration partitioning using a tracer-aided model

Evapotranspiration (ET) partitioning is a growing field of research in hydrology due to the significant fraction of watershed water loss it represents. The use of tracer-aided models has improved understanding of watershed processes, and has significant potential for identifying time-variable partitioning of evaporation (E) from ET. A tracer-aided model was used to establish a time-series of E/ET using differences in riverine δ18O and δ2H in four northern Canadian watersheds (lower Nelson River, Manitoba, Canada). On average E/ET follows a parabolic trend ranging from 0.7 in the spring and autumn to 0.15 (three watersheds) and 0.5 (fourth watershed) during the summer growing season. In the fourth watershed wetlands and shrubs dominate land cover. During the summer, E/ET ratios are highest in wetlands for three watersheds (10% higher than unsaturated soil storage), while lowest for the fourth watershed (20% lower than unsaturated soil storage). Uncertainty of the ET partition parameters is strongly influenced by storage volumes, with large storage volumes increasing partition uncertainty. In addition, higher simulated soil moisture increases estimated E/ET. Although unsaturated soil storage accounts for larger surface areas in these watersheds than wetlands, riverine isotopic composition is more strongly affected by E from wetlands. Comparisons of E/ET to measurement-intensive studies in similar ecoregions indicate that the methodology proposed here adequately partitions ET.

Comparison of δ18O analyses on individual planktic foraminifer (Orbulina universa) shells by SIMS and gas-source mass spectrometry

Abstract The oxygen isotope (δ18O) compositions of final chamber fragments of individual shells of the planktic foraminifer Orbulina universa were measured in situ via secondary ion mass spectrometry (SIMS) and by traditional gas-source mass spectrometry (GSMS) entailing acid digestion of sampled calcite. The paired SIMS-GSMS analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core (Holocene) as well as shells grown in laboratory culture. Multiple iterations of SIMS-GSMS analyses were conducted on final chamber fragments treated with a variety of cleaning protocols. The series of paired analyses yielded an average SIMS-GSMS δ18O offset (Δ18OSIMS-GSMS) of −0.9 ± 0.1‰ (±2 SE). The volume of material analyzed in 10-μm SIMS spots is ~105 times smaller than that analyzed by GSMS; hence, the extent to which these Δ18OSIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSMS δ18O difference include sample-standard mismatch by SIMS, differences in standardization of SIMS and GSMS, and non-calcite contaminants in samples. Although the two datasets are consistently offset, SIMS values reproduce inter-shell δ18O variability delineated by shell fragment GSMS values. This strong positive covariance proved useful for bringing the two datasets into agreement (i.e. Δ18OSIMS-GSMS = 0), and confirms that SIMS-based foraminifer δ18O values record changes in calcification temperature and/or δ18O of seawater. Whether shells of foraminifer taxa with differing microcrystalline structures, chemical composition, and/or preservation histories register a similar Δ18OSIMS-GSMS value is a subject of ongoing testing.

Migration of the Kuroshio Extension in the Northwest Pacific since the Last Glacial Maximum

The δ18O values of planktic and benthic foraminifera and faunal assemblage of planktic foraminifera were investigated at a Northwest Pacific site (32°16′N, 158°13′E; 2514 m water depth) to understand the oceanographic changes in the Kuroshio Extension (KE) region over the past 23 kyr. The δ18O differences between species and the relative abundances of the dominant planktic foraminiferal species have not changed greatly over the study period, suggesting that the subarctic boundary and the subarctic–subtropical transition zone have remained north of study site, while the study site itself has been under the influence of KE. Nevertheless, increased abundances of foraminiferal taxa that populated the subarctic–subtropical transition zone during the last glacial maximum (LGM) indicate the input of transition zone water through the KE, manifest as meandering cold-core ring eddies generated along the KE. Since this unstable surface environment is most pronounced along the main axis of the KE, it could have been induced by the southward displacement of the KE main axis to the vicinity of the study site, in tandem with the overall southward displacement of the Kuroshio–Oyashio Extension system during the LGM. The input of transition-zone water mass has also increased during the deglacial Northern Hemisphere cold intervals (i.e., Heinrich Stadial 1 and the Younger Dryas), suggesting that the strengthened surface wind system regulated the surface ocean condition. This demonstrates that the northern high latitudes are the dominant control on post-glacial oceanographic changes in the Northwest Pacific KE region.

Distinct responses of East Asian and Indian summer monsoons to astronomical insolation during Marine Isotope Stages 5c and 5e

Loess and stalagmite records in China show different responses of Asian monsoon to the astronomical insolation during Marine Isotope Stage (MIS) 5c and 5e, raising the question whether the amplitudes of the whole Asian monsoon are linearly controlled by the astronomical summer insolation changes and whether the regional differences exist. In this study, a set of numerical experiments are conducted for these stages to examine the sensitivity of Asian summer monsoon to changes in the astronomical parameters. The results highlight a complicated response changing with regions and signals. The East Asian monsoon is remarkably stronger during MIS-5e, which is characterized by southerly winds, primarily due to the anomalous anticyclone over northwestern Pacific. The intensified East Asian monsoon leads to less precipitation in the eastern China and Japan but more in the further north and northwest. However, the Indian monsoon circulation does not vary significantly while the precipitation over Arabian Sea is significantly promoted by the upward air motion. Change in water vapor sources of Indian and Pacific Oceans qualitatively explains the observed differences in stalagmite δ18O records, implying that these proxy changes are primarily controlled by the water vapor delivery from oceans, at least on the astronomical timescale.

Environmental controls on stable isotopes of precipitation in Lanzhou, China: An enhanced network at city scale

Stable hydrogen and oxygen isotopes in precipitation are very sensitive to environmental changes, and can record evolution of water cycle. The Lanzhou city in northwestern China is jointly influenced by the monsoon and westerlies, which is considered as a vital platform to investigate the moisture regime for this region. Since 2011, an observation network of stable isotopes in precipitation was established across the city, and four stations were included in the network. In 2013, six more sampling stations were added, and the enhanced network might provide more meaningful information on spatial incoherence and synoptic process. This study focused on the variations of stable isotopes (δ18O and δD) in precipitation and the environmental controls based on the 1432 samples in this enhanced network from April 2011 to October 2014. The results showed that the precipitation isotopes had great spatial diversity, and the neighboring stations may present large difference in δD and δ18O. Based on the observation at ten sampling sites, an isoscape in precipitation was calculated, and the method is useful to produce isoscape for small domains. The temperature effect and amount effect was reconsidered based on the dataset. Taking meteorological parameters (temperature, precipitation amount, relative humidity, water vapor pressure and dew point temperature) as variables in a multi-linear regression, the result of coefficients for these meteorological parameters were calculated. Some cases were also involved in this study, and the isotopic characteristics during one event or continuous days were used to understand the environmental controls on precipitation isotopes.

Variation in subsurface water temperature and its link to the Kuroshio Current in the Okinawa Trough during the last 38.5 kyr

An 820.4-cm long piston core (YK12-15 PC-01) was obtained from site OT-1B (28°07.15′N, 127°11.97′E, 1050 m water depth), located in the middle of the Okinawa Trough in the southern East China Sea (ECS). For core YK12-15 PC-01, oxygen and carbon isotopes (δ18O and δ13C) of planktic foraminifera Globigerinoides ruber sensu stricto (250–355 μm size fraction) and Globorotalia inflata (>250 μm size fraction) together with total organic carbon (TOC), nitrogen content (TN), and δ13C of organic matter were analyzed for the last 38.5 thousand years (kyr). The δ18O records of G. ruber (δ18Oruber) and G. inflata (δ18Oinflata) were used to estimate the temperature/salinity of the surface and subsurface waters, respectively. In addition, we estimated the vertical temperature gradient between the surface and the subsurface based on the difference in δ18O between G. ruber and G. inflata (Δδ18Oruber-inflata). The δ18Oinflata and Δδ18Oruber-inflata indicate millennial-scale variations in association with the oscillations in Antarctic temperatures during the Marine Isotope Stage (MIS) 3. These results suggest that variations in subsurface temperature and vertical temperature gradient in the upper water column in the ECS were closely related to the Atlantic bipolar seesaw during MIS 3. The subsurface temperature in the ECS was higher at the beginning of Greenland stadials when the Antarctic temperatures showed the lowest values. The warming of the subsurface is likely attributed to the stronger Kuroshio transport. High peaks in the TOC and TN records coincide with the low peaks in δ18Oinflata (high temperature), suggesting higher primary productivity, which is in good agreement with the interpretation of the intensified Kuroshio. However, the subsurface warming was not recognized during Herinch events, indicating different temperature responses in the upper water column from other stadials.

Revisiting the Ceara Rise, equatorial Atlantic Ocean: isotope stratigraphy of ODP Leg 154 from 0 to 5 Ma

Abstract. Isotope stratigraphy has become the method of choice for investigating both past ocean temperatures and global ice volume. Lisiecki and Raymo (2005) published a stacked record of 57 globally distributed benthic δ18O records versus age (LR04 stack). In this study LR04 is compared to high-resolution records collected at all of the sites drilled during ODP Leg 154 on the Ceara Rise, in the western equatorial Atlantic Ocean. Newly developed software is used to check data splices of the Ceara Rise sites and better align out-of-splice data with in-splice data. Core images recovered from core table photos are depth and age scaled and greatly assist in the data analysis. The entire splices of ODP sites 925, 926, 927, 928 and 929 were reviewed. Most changes were minor although several were large enough to affect age models based on orbital tuning. A Ceara Rise composite record of benthic δ18O is out of sync with LR04 between 1.80 and 1.90 Ma, where LR04 exhibits two maxima but Ceara Rise data contain only one. The interval between 4.0 and 4.5 Ma in the Ceara Rise compilation is decidedly different from LR04, reflecting both the low amplitude of the signal over this interval and the limited amount of data available for the LR04 stack. A regional difference in benthic δ18O of 0.2 ‰ relative to LR04 was found. Independent tuning of Site 926 images and physical property data to the Laskar et al. (2004) orbital solution and integration of available benthic stable isotope data from the Ceara Rise provides a new regional reference section for the equatorial Atlantic covering the last 5 million years.

Movement, connectivity and population structure of the intertidal fish Lipophrys pholis as revealed by otolith oxygen and carbon stable isotopes

ABSTRACTThe shanny Lipophrys pholis is an intertidal fish commonly found in Portuguese coastal waters. Spawning takes place from early autumn to mid spring, after which demersal eggs hatch and larvae disperse along the coast. Two to three months later, young juveniles return to the tide pools to settle. However, information on fish movement, habitat connectivity and population structure is scarce for this species. One hundred and twenty early juveniles (16–35 mm) were collected in April 2014 from six rocky beaches along the western and south Portuguese coasts (Agudela, Cabo do Mundo, Boa Nova, Peniche, Sines and Olhos de Água). δ18O and δ13C were determined by isotope-ratio mass spectrometry. Data were analysed to determine whether isotopic signatures could be used to assess the degree of separation between individuals collected from different locations. Mean δ13C and δ18O values ranged from −0.02‰ to 1.14‰ and −7.77‰ to −6.62‰, respectively. Both seawater temperature and salinity caused differences in otolith δ18O among the four main sampling areas. The variation among areas in δ13C was most likely related to slight differences in the diet, growth and metabolism of fish. The distinct isotopic signatures, at least for the northern and central areas, suggested low levels of connectivity across large spatial scales during the juvenile stage. Furthermore, similar isotopic signatures within the same area indicated some degree of larval oceanic retention at short spatial scales. This study suggests that stable isotope records in otoliths could provide information about the home residency, movements and habitat connectivity of intertidal fishes.

Stable oxygen isotope analysis confirms natural recruitment of Lake Michigan-origin lake trout (Salvelinus namaycush) to the adult life stage

Following Lake Michigan’s lake trout (Salvelinus namaycush) population collapse in the mid-1900s, fisheries managers began stocking marked juveniles in 1965 in an effort to restore the population. However, there has been little evidence of naturally produced juveniles recruiting to the adult life stage, but recent collections of unmarked adults in Illinois waters of Lake Michigan suggest otherwise. Here, we provide stable isotopic evidence that confirms recruitment to the adult life stage is occurring in southwestern Lake Michigan. Our results indicate substantial differences in δ18O and δ13C values between hatchery-reared juvenile lake trout and the putative wild adults, effectively eliminating the possibility that unmarked adults were erroneously miss-clipped as hatchery-reared juveniles. We also found significant differences in δ18O signatures between lakes Michigan and Huron and successfully classified over 90% of putative wild adult lake trout as Lake Michigan-origin. Trace element analyses were less effective in distinguishing fish from lakes Huron and Michigan than otolith δ18O. These findings indicate that Lake Michigan’s lake trout restoration program is at least partially effective. Further investigation is required to determine what factors may be facilitating natural reproduction and adult recruitment in southwestern Lake Michigan.

Authigenic Nd isotope record of North Pacific Intermediate Water formation and boundary exchange on the Bering Slope

Abstract The Bering Sea is a potential location for the formation of the North Pacific Intermediate Water (NPIW), which drives the global ocean circulation as a counterpart to the North Atlantic Deep Water (NADW). To evaluate the NPIW-NADW seesaw hypothesis, we reconstructed the long-term variation of the bottom water Nd isotopic composition at site U1345 on the Bering Slope by extracting authigenic Fe-Mn oxyhydroxide from bulk sediments. We examined six different extractions in order to ensure that authentic seawater composition is recovered. For Bering Slope sediments whose typical carbonate content is less than 5% (average 2%), the most reliable results are obtained if the decarbonation step is omitted and a low reagent-to-sediment ratio is adopted. The reconstructed authigenic eNd record for the last 520 kyr exhibits large temporal variations depending on whether the NPIW formation or the boundary exchange process is dominant. Periods of radiogenic eNd can be attributed to NPIW formation triggered by brine rejection, as evidenced by the difference in δ18O of benthic foraminifera between sites (Δδ18Obf), high % sea-ice related diatoms, and low abundance of Bulimina aff. Exilis (low-oxygen deep fauna). Diminished supply of unradiogenic Nd from boundary exchange seems to intensify these radiogenic peaks. On the other hand, the unradiogenic eNd intervals can be attributed to stagnant bottom water conditions, as can be deduced from the Δδ18Obf values, low % sea-ice related diatoms, abundant B. aff. Exilis, and laminations. When there is no NPIW formation, the continental margin sediments are exposed to boundary exchange for a longer period of time, leading to release of unradiogenic Nd. The mid-MIS 6 and mid-MIS 5 are exceptions in that NPIW formation occurred yet the eNd compositions are unradiogenic. NPIW formation and cold climate (closed Bering Strait) are not always correlated. Comparison against eNd records of the South Atlantic suggests only an ambiguous NPIW-NADW seesaw for the last 200 kyr.

Oxygen isotope thermometry using quartz inclusions in garnet

AbstractOxygen isotope ratios of quartz inclusions (QI) within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analysed and used to determine metamorphic temperatures. Primary QI for eight of 12 samples have δ18O values significantly lower than matrix quartz (MQ). The primary QI retain δ18O values representative of thermal conditions during garnet crystallization, whereas the δ18O values of MQ were raised by diffusive exchange with other matrix minerals (e.g. mica and feldspar) during cooling. The δ18O differences between QI and MQ show that garnet (a mineral with slow diffusion of oxygen) can armour QI from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ18O in MQ and QI can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying QI that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ18O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in QI, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of QI preserve primary δ18O and 39% were associated with features that were linked to reset δ18O values. If δ18O in garnet is homogeneous and inclusions are removed, laser‐fluorination δ18O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ18O(Grt) profiles measured in situ by ion microprobe show no δ18O zonation. Almandine–rich garnet (Alm60–75) from each sample was measured by laser‐fluorination mass‐spectrometry (LF‐MS) for δ18O and compared with ion microprobe measurements of δ18O in QI for thermometry. The Δ18O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640–740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625–800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the anorthosite–mangerite–charnockite–granite suite in the central Adirondacks and yield temperatures of 640–665 °C, ~100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post‐peak) crystallization of garnet on the retrograde path.