ALBERT

Description: 〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Laser ablation multi‐collector‐inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) has become a valuable tool for the 〈italic toggle="no"〉in situ〈/italic〉 measurement of the boron isotope composition of geological samples at high (tens to hundreds of μm) spatial resolution. That said, this application suffers from significant analytical challenges. We focus in this study on the underlying processes of two of the main causes for inaccuracies using this technique. We provide empirical evidence that not only Ca ions (Sadekov 〈italic〉et al〈/italic〉. 2019, Standish 〈italic〉et al〈/italic〉. 2019, Evans 〈italic〉et al〈/italic〉. 2021) but also Ar ions, that are reflected within the flight tube of the mass spectrometer, are the source for previously reported issues with spectral baselines. We also address the impact of plasma conditions on the instrumental mass fractionation as a source for matrix‐ and mass‐load‐related analytical biases. Comparing experimental data with the results of a dedicated release and diffusion model (RDM) we estimate that a close to complete (~ 97%) release of boron from the sample aerosol is needed to allow for consistently accurate LA boron isotope measurement results without the need for corrections.〈/p〉

Description: Key Points: 〈list list-type="bullet" id="ggr12500-list-0101"〉 〈list-item〉〈p〉Two separate main sources for inaccuracy of boron isotope measurements by laser ablation: B isotope fractionation in the ICP and the known scattered ion baseline problem.〈/p〉〈/list-item〉 〈list-item〉〈p〉Boron isotope fractionation in the ICP varies systematically with plasma condition (NAI).〈/p〉〈/list-item〉 〈list-item〉〈p〉Behaviour of B isotopes within the ICP simulated by a release and diffusion model (RDM).〈/p〉〈/list-item〉 〈/list〉 〈boxed-text position="anchor" content-type="graphic" id="ggr12511-blkfxd-0001" xml:lang="en"〉 〈graphic position="anchor" id="jats-graphic-1" xlink:href="urn:x-wiley:16394488:media:ggr12511:ggr12511-toc-0001"〉 〈/graphic〉 〈/boxed-text〉 〈/p〉

Description: Over the past decades, coralline algae have increasingly been used as archives of palaeoclimate information due to their seasonal growth bands and their vast distribution from high latitudes to the tropics. Traditionally, these reconstructions have been performed mainly on high latitude species, limiting the geographical area of their potential use. Here we assess the use of temperate crustose fossil coralline algae from shallow water habitats for palaeoenvironmental reconstruction to generate records of past climate change. We determine the potential of three different species of coralline algae, Lithothamnion minervae, Lithophyllum stictaeforme and Mesophyllum philippii, with different growth patterns, as archives for pH (δ11B) and temperature (Mg/Ca) reconstruction in the Mediterranean Sea. Mg concentration is driven by temperature but modulated by growth rate, which is controlled by species‐specific and intraspecific growth patterns. L. minervae is a good temperature recorder, showing a moderate warming trend in specimens from 11.37 cal ka BP (from 14.2 ± 0.4°C to 14.9 ± 0.15°C) to today. In contrast to Mg, all genera showed consistent values of boron isotopes (δ11B) suggesting a common control on boron incorporation. The recorded δ11B in modern and fossil coralline specimens is in agreement with literature data about early Holocene pH, opening new perspectives of coralline‐based, high‐resolution pH reconstructions in deep time.

Description: Highlights • Improved understanding of the behaviour of instrumental mass fractionation (IMF). • The effect of matrix elements on IMF is largely associated with plasma conditions that can be quantified with the NAI. • Matrix effects can be systematically and significantly attenuated by tuning of instrumental operating parameters. • A matrix tolerance plasma state is defined for stable barium isotope analysis. • The suggested analytical protocol is expected to be applicable to other stable isotope measurements with MC-ICP-MS. Abstract Stable barium isotope measurements with multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) remain an analytical challenge and can be considerably affected by the presence of matrix elements, even when applying double spiking. Therefore significant efforts were invested in previous studies to develop efficient barium purification methods. However, due to the high variability in matrix/barium ratios for diverse sample matrices, potential matrix effects can still not be excluded. While a lot of effort has been invested into improving the chemical separation protocols, the impact of plasma conditions on the accuracy and precision of stable isotope measurements has rarely been considered. Here we present a systematic investigation of the relationship between plasma conditions, instrumental mass fractionation (IMF) and impurity (i.e. matrix) concentrations. The Normalised Ar Index (NAI) and Matrix-Ar Index (MA) are used to quantify MC-ICP-MS plasma conditions and plasma mass loading, respectively. Our results show that the effect of matrix elements on IMF is largely linked to plasma conditions (i.e. NAI) and behaves as a linear function of mass loading (i.e. MA). Accordingly, the matrix effects can be significantly attenuated by increasing the NAI thereby minimising the risk of plasma “over-loading”. The improved understanding of the behaviour of the matrix-induced IMF allows us to define a matrix tolerance plasma state for barium isotope analysis. The accuracy of this recommended method is further assessed by analyses of two well-studied reference materials, the GEOTRACES seawater reference sample SAFe D2 and the carbonate reference material JCp-1. We expect that the analytical protocol described in this study is applicable not only to barium isotope analysis, but also to a wide range of other stable isotope measurements with MC-ICP-MS.

Description: Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and δ18O are important tools for quantitative analysis of past climate variabilities. However, post-depositional alteration of coral aragonite, particularly early diagenesis, restrict the accuracy of calibrated proxies even on young corals. Considering the diagenetic effects, we present new Mid to Late Holocene SST reconstructions on well-dated (U/Th: ∼70 yr to 5.4 ka) fossil Porites sp. collected from the Society Islands, French Polynesia. For few corals, quality pre-screening routines revealed the presence of secondary aragonite needles inside primary pore space, resulting in a mean increase in Sr/Ca ratios between 5-30%, in contrast to the massive skeletal parts. Characterized by a Sr/Ca above 10 mmol/mol, we interpret this value as the threshold between diagenetically altered and unaltered coral material. At a high-resolution, observed intra-skeletal variability of 5.4 to 9.9 mmol/mol probably reflects the physiological control of corals over their trace metal uptake, and individual variations controlled by CaCO3– precipitation rates. Overall, the Sr/Ca, U/Ca and δ18O trends are well correlated, but we observed a significant offset up to ± 7°C among the proxies on derived palaeo-SST estimates. It appears that the related alteration process tends to amplify temperature extremes, resulting in increased SST-U/Ca and SST-Sr/Ca gradients, and consequently their apparent temperature sensitivities. A relative SST reconstruction is still feasible by normalizing our records to their individual mean value defined as ΔSST. This approach shows that ΔSST records derived from different proxies agree with an amplitudinal variability of up to ± 2°C with respect to their Holocene mean value. Higher ΔSST values than the mean SSTs (Holocene warm periods) were recorded from ∼1.8 to ∼2.8 ka (Interval I), ∼3.7 to 4.0 ka (Interval III) and before ∼5 ka, while lower ΔSST values (Holocene cold periods, Interval II and IV) were recorded in between. The ensuing SST periodicity of ∼1.5 ka in the Society Islands record is in line with the solar activity reconstructed from 10Be and 14C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene.

Description: Boron isotope (δ11B) analysis using bulk foraminifera samples is a widely used method to reconstruct paleo sea water pH conditions. Although, these analyses exhibit high analytical precision, short term information is lost due to the pooling of tests with distinct and diverse boron isotope signatures resulting in average values for the time interval encompassed in the sample. Here we present and assess the analysis of δ11B of individual foraminifera by means of Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) to obtain seasonal variability patterns and to test the limits of precision of LA-MC-ICP-MS on the planktonic foraminifera Orbulina universa. The results show that relative seasonal differences (of ∼11 ‰) can be captured from either uncleaned or cleaned individual O. universa tests with an average precision of ±2.9 ‰ (2 SE). The δ11B variability among foraminifera representing the same season is on average 7.4 ‰ (2 SD) irrespective of cleaning state. With our approach, analyses on oxidatively cleaned O. universa do not require the use of a matrix matched standard to obtain B isotope values in the range of those expected for solution multi-specimen analyses from determining local pH. Our results are useful for considering the potential spread caused by foraminifera vital effects and for obtaining information of seasonal ranges of pH and possible bias related to seasonality hidden within conventional solution based δ11B analyses.

Description: Predator loss and climate change are hallmarks of the Anthropocene yet their interactive effects are largely unknown. Here, we show that massive calcareous reefs, built slowly by the alga Clathromorphum nereostratum over centuries to millennia, are now declining because of the emerging interplay between these two processes. Such reefs, the structural base of Aleutian kelp forests, are rapidly eroding because of overgrazing by herbivores. Historical reconstructions and experiments reveal that overgrazing was initiated by the loss of sea otters, Enhydra lutris (which gave rise to herbivores capable of causing bioerosion), and then accelerated with ocean warming and acidification (which increased per capita lethal grazing by 34 to 60% compared with preindustrial times). Thus, keystone predators can mediate the ways in which climate effects emerge in nature and the pace with which they alter ecosystems.

Description: Highlights • Holocene cold-water coral mound formation started non-synchronous in Belgica province. • Coral mounds and slope sediments record changes in intermediate water mass dynamics. • Increased turbulent bottom currents steered slope erosion and mound formation. • Internal waves at the ENAW-MOW boundary enhance energy supply and particle flux. • Transition zone between the ENAW-MOW shifted 250 m upslope during the last deglacial. Abstract Turbulent bottom currents significantly influence the formation of cold-water coral mounds and sedimentation processes on continental slopes. Combining records from coral mounds and adjacent slope sediments therefore provide an unprecedented palaeo-archive to understand past variations of intermediate water-mass dynamics. Here, we present coral ages from coral mounds of the Belgica province (Porcupine Seabight, NE Atlantic), which indicate a non-synchronous Holocene re-activation in mound formation suggested by a temporal offset of ∼2.7 kyr between the deep (start: ∼11.3 ka BP at 950 m depth) and shallow (start: ∼8.6 ka BP at 700 m depth) mounds. A similar depth-dependent pattern is revealed in the slope sediments close to these mounds that become progressively younger from 22.1 ka BP at 990 m to 12.2 ka BP at 740 m depth (based on core-top ages). We suggest that the observed changes are the consequence of enhanced bottom-water hydrodynamics, caused by internal waves associated to the re-invigoration of the Mediterranean Outflow Water (MOW) and the development of a transition zone (TZ) between the MOW and the overlying Eastern North Atlantic Water (ENAW), which established during the last deglacial. These highly energetic conditions induced erosion adjacent to the Belgica mounds and supported the re-initiation of mound formation by increasing food and sediment fluxes. The striking depth-dependent patterns are likely linked to a shift of the ENAW-MOW-TZ, moving the level of maximum energy ∼250 m upslope since the onset of the last deglaciation.

Description: Highlights • Coral reef responses to slow-rate, low-amplitude, high-frequency sea-level changes. • Continuous reef development over the last 6000 years in optimal conditions. • Mid-late Holocene coral assemblages similar to their modern counterparts. • Facies distribution governed by variations in accommodation space. • Prominent role played by reef deposits in the formation and shaping of modern islands. Geological records of coastal system evolution during past higher and/or rising sea levels provide an important baseline for developing projections regarding the response of modern coastal systems to future sea-level rise. The mid-late Holocene corresponds to the most recent window into natural variability prior to the Anthropocene and involves slow-rate and low-amplitude sea-level changes that were mostly governed by a limited glacio-eustatic contribution, most likely sourced from Antarctica, and ‘glacial isostatic adjustment’ processes. This paper documents in unprecedented detail the response of coral reefs and coastal systems to changing accommodation space in relation to mid-late Holocene sea-level changes in French Polynesia. The sea-level curve that underpins this study has a global significance and documents a single short-lived sea-level highstand between 4.10 and 3.40 kyr BP. The amplitude of the highstand is less than one metre, within the range of the predicted sea level at the end of the current century. The reported relative sea-level changes are characterized by slow rates ranging from a few tens of millimetres per year to up to 2.5 mm/yr and by significant sea-level stability (stillstands) lasting more than a century and up to 250 years, defining a step-like pattern. Sea-level variability probably driven by climatic oscillations on interannual to millennial time scales is evidenced during the entire time window. The detailed reconstruction of reef development over the last 6000 years brings valuable information regarding coral reef dynamics and coastal processes during periods of higher sea level and wave energy regimes. The persistence of stable and optimal depositional environments over the last 6000 years is demonstrated by the constant overall composition and diversity of reef communities and the almost continuous development of coral microatolls. The facies distribution as well as the lateral extension and shift of facies belts have been governed by variations in accommodation space, which are controlled by relative sea-level changes and antecedent topography. The widespread development of mid-late Holocene reef deposits in coastal areas suggests that they have played a prominent role in global processes related to the formation and shaping of modern islands.

Description: Abstract Robust chronologies and time equivalent tephra markers are essential to better understand spatial palaeoenvironmental response to past abrupt climatic changes. Identification of well-dated and widely dispersed volcanic ash by tephra and cryptotephra (microscopic volcanic ash) provide time synchronous tie-points and strongly reduce chronological uncertainties. Here, we present the major, minor and trace element analyses of cryptotephra shards in the Dead Sea Deep Drilling sedimentary record (DSDDP 5017-1A) matching the Campanian Ignimbrite (CI). This geochemical identification expands the know dispersal range of the CI to the southeastern Mediterranean, over 2,300 km from the volcanic source. Due to the CI eruption occurring near-synchronous with North Atlantic ice surge of Heinrich Event 4 (HE4), this tephra provides insights into regional responses to large-scale climatic change in the Mediterranean. In the Dead Sea, the CI layer is associated with wetter climatic conditions. This contrasts with the contemporaneous occurrence of the CI deposition and dry conditions in the northern and western Mediterranean suggesting a possible climate time-transgressive expansion of HE4. Our finding underscores the temporal and spatial complexity of regional climate responses and emphasises the importance of tephra as a time marker for studying large-scale climatic changes verses regional variations.