The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern ...hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to ∼39°N until 2.4 ± 0.14 Ma, at least 180 ka after the onset of major IRD deposition at 2.72 Ma. To address this problem, we present the first detailed analysis of the geochemical provenance of individual sand-sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 and 100 (∼2.72–2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150 μm) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100 (2.52 Ma), the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary-magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb-isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma.
•We studied flux and provenance of Pliocene North Atlantic ice-rafted debris, IRD.•Provenance determined using laser ablation Pb-isotope analyses of single feldspars.•Find that onset of major ice-rafting at 2.72 Ma sourced from Greenland and Europe.•Propose North America not a significant source of IRD to North Atlantic until 2.64 Ma.•That onset of major northern hemisphere glaciation occurred at 2.64 Ma.
Deposited within the Indus–Tsangpo suture zone, the Cenozoic Indus Basin sedimentary rocks have been interpreted to hold evidence that may constrain the timing of India–Eurasia collision, a ...conclusion challenged by data presented here. The Eurasian derived 50.8–51
Ma Chogdo Formation was previously considered to overlie Indian Plate marine sedimentary rocks in sedimentary contact, thus constraining the timing of collision as having occurred by this time. Using isotopic analysis (U–Pb dating on detrital zircons, Ar–Ar dating on detrital white mica, Sm–Nd analyses on detrital apatite), sandstone and conglomerate petrography, mudstone geochemistry, facies analysis and geological mapping to characterize and correlate the formations of the Indus Basin Sedimentary rocks, we review the nature of these contacts and the identification and correlation of the formations. Our results reveal that previously interpreted stratigraphic contacts identifying Chogdo Formation unconformably overlying Indian plate sedimentary rocks are incorrect. Rather, we suggest that the inaccuracy of previous interpretations is most likely a result of Formation misidentification and thus cannot be used to constrain the timing of India–Asia collision.
Warm ocean processes and carbon cycling in the Eocene John, Eleanor H.; Pearson, Paul N.; Coxall, Helen K. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
10/2013, Letnik:
371, Številka:
2001
Journal Article
Recenzirano
Odprti dostop
Sea surface and subsurface temperatures over large parts of the ocean during the Eocene epoch (55.533.7Ma) exceeded modern values by several degrees, which must have affected a number of oceanic ...processes. Here, we focus on the effect of elevated water column temperatures on the efficiency of the biological pump, particularly in relation to carbon and nutrient cycling. We use stable isotope values from exceptionally well-preserved planktonic foraminiferal calcite from Tanzania and Mexico to reconstruct vertical carbon isotope gradients in the upper water column, exploiting the fact that individual species lived and calcified at different depths. The oxygen isotope ratios of different species' tests are used to estimate the temperature of calcification, which we converted to absolute depths using Eocene temperature profiles generated by general circulation models. This approach, along with potential pitfalls, is illustrated using data from modern core-top assemblages from the same area. Our results indicate that, during the Early and Middle Eocene, carbon isotope gradients were steeper (and larger) through the upper thermocline than in the modern ocean. This is consistent with a shallower average depth of organic matter remineralization and supports previously proposed hypotheses that invoke high metabolic rates in a warm Eocene ocean, leading to more efficient recycling of organic matter and reduced burial rates of organic carbon.
Coral Sr‐U thermometry DeCarlo, Thomas M.; Gaetani, Glenn A.; Cohen, Anne L. ...
Paleoceanography,
June 2016, 2016-06-00, 20160601, Letnik:
31, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Coral skeletons archive past climate variability with unrivaled temporal resolution. However, extraction of accurate temperature information from coral skeletons has been limited by “vital effects,” ...which confound, and sometimes override, the temperature dependence of geochemical proxies. We present a new approach to coral paleothermometry based on results of abiogenic precipitation experiments interpreted within a framework provided by a quantitative model of the coral biomineralization process. DeCarlo et al. (2015a) investigated temperature and carbonate chemistry controls on abiogenic partitioning of Sr/Ca and U/Ca between aragonite and seawater and modeled the sensitivity of skeletal composition to processes occurring at the site of calcification. The model predicts that temperature can be accurately reconstructed from coral skeleton by combining Sr/Ca and U/Ca ratios into a new proxy, which we refer to hereafter as the Sr‐U thermometer. Here we test the model predictions with measured Sr/Ca and U/Ca ratios of 14 Porites sp. corals collected from the tropical Pacific Ocean and the Red Sea, with a subset also analyzed using the boron isotope (δ11B) pH proxy. Observed relationships among Sr/Ca, U/Ca, and δ11B agree with model predictions, indicating that the model accounts for the key features of the coral biomineralization process. By calibrating to instrumental temperature records, we show that Sr‐U captures 93% of mean annual temperature variability (26–30°C) and has a standard deviation of prediction of 0.5°C, compared to 1°C using Sr/Ca alone. The Sr‐U thermometer may offer significantly improved reliability for reconstructing past ocean temperatures from coral skeletons.
Key Points
Coral biomineralization confounds geochemical temperature proxies based on single element/Ca ratios
U/Ca ratios track the calcifying fluid variations that distort the temperature dependence of Sr/Ca
Coral Sr/Ca and U/Ca ratios used in tandem improve accuracy of seawater temperature reconstructions
Boron isotope ratios, as measured in planktic foraminifera, can be a useful tracer of past ocean pH, and hence help to discern the concentration of CO2 in the ancient atmosphere. However, different ...species of planktic foraminifera demonstrate different patterns of boron isotope variation with ambient seawater pH. Therefore when applying the proxy to questions in the geological past, species-specific calibrations are preferable. Beyond the evolutionary history of a calibrated species, we must rely on our understanding of the causes of the observed “vital effects” in the modern ocean, and the applicability of that understanding to extinct species. Here we present a new open-ocean calibration of the planktic foraminifera Orbulina universa, measured via Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS). Unlike other symbiont-bearing foraminifera, O. universa record a δ11B (and hence pH) that is lower than its surrounding seawater, but with a pH-sensitivity roughly equal to that of aqueous borate ion. We discuss the significance of this for application of the boron isotope proxy in deep time, with recommendations for best practice and future research directions.
•A new, well-constrained boron isotope calibration for O. universa.•O. universa records lower pH than ambient seawater, which we suggest is due to living at depth.•We give recommendations for accounting for vital effects in extinct species.
Global ice volume (sea level) and deep‐sea temperature are key measures of Earth's climatic state. We synthesize evidence for multi‐centennial to millennial ice‐volume and deep‐sea temperature ...variations over the past 40 million years, which encompass the early glaciation of Antarctica at ∼34 million years ago (Ma), the end of the Middle Miocene Climate Optimum, and the descent into bipolar glaciation from ∼3.4 Ma. We compare different sea‐level and deep‐water temperature reconstructions to build a resource for validating long‐term numerical model‐based approaches. We present: (a) a new template synthesis of ice‐volume and deep‐sea temperature variations for the past 5.3 million years; (b) an extended template for the interval between 5.3 and 40 Ma; and (c) a discussion of uncertainties and limitations. We highlight key issues associated with glacial state changes in the geological record from 40 Ma to present that require attention in further research. These include offsets between calibration‐sensitive versus thermodynamically guided deep‐sea paleothermometry proxy measurements; a conundrum related to the magnitudes of sea‐level and deep‐sea temperature change at the Eocene‐Oligocene transition at 34 Ma; a discrepancy in deep‐sea temperature levels during the Middle Miocene; and a hitherto unquantified non‐linear reduction of glacial deep‐sea temperatures through the past 3.4 million years toward a near‐freezing deep‐sea temperature asymptote, while sea level stepped down in a more uniform manner. Uncertainties in proxy‐based reconstructions hinder further distinction of “reality” among reconstructions. It seems more promising to further narrow this using three‐dimensional ice‐sheet models with realistic ice‐climate‐ocean‐topography‐lithosphere coupling, as computational capacities improve.
Plain Language Summary
Global ice volume (hence, sea level) and deep‐sea temperature are important measures of Earth's climatic state. To better understand Earth's climate cycles in response to its orbitally driven insolation cycles, we evaluate and synthesize evidence for ice‐volume (sea‐level) and deep‐sea temperature variations at multi‐centennial to millennial resolution throughout the last 40 million years. These last 40 million years encompass the major build‐up of Antarctic glaciation from about 34 million years ago, and development of extensive Northern Hemisphere ice sheets from about 3.4 million years ago. We present a new template synthesis of ice‐volume (sea‐level) and deep‐sea temperature for the past 5.3 million years, with extension through the interval between 5.3 and 40 Ma with wider uncertainties. We also highlight a number of remaining questions about major climate transitions, including the early glaciation history of Antarctica, the end of the so‐called Middle Miocene Climate Optimum from about ∼14.5 Ma, and the descent over the past several million years into conditions with extensive ice‐age maxima in both hemispheres.
Key Points
New synthesis of sea level and deep‐sea temperature over 40 million years, at millennial resolution, across seven different methods
Discrepancies between Eocene‐Oligocene and Middle Miocene sea‐level reconstructions are highlighted for focussed future research
Plio‐Pleistocene glacial deep‐sea temperature asymptoted to a freezing limit by ∼1.25–0.9 Ma; glacial sea‐level minima decreased ∼linearly
Climate sensitivity represents the global mean temperature change caused by changes in the radiative balance of climate; it is studied for both present future (actuo) and past (paleo) climate ...variations, with the former based on instrumental records and or various types of model simulations. Paleo-estimates are often considered informative for assessments of actuo-climate change caused by anthropogenic greenhouse forcing, but this utility remains debated because of concerns about the impacts of uncertainties, assumptions, and incomplete knowledge about controlling mechanisms in the dynamic climate system, with its multiple interacting feedbacks and their potential dependence on the climate background state. This is exacerbated by the need to assess actuo- and paleoclimate sensitivity over different timescales, with different drivers, and with different (data and or model) limitations. Here, we visualize these impacts with idealized representations that graphically illustrate the nature of time-dependent actuo- and paleoclimate sensitivity estimates, evaluating the strengths, weaknesses, agreements, and differences of the two approaches. We also highlight priorities for future research to improve the use of paleo-estimates in evaluations of current climate change.
Deep water formation in the North Atlantic and Southern Ocean is widely thought to influence deglacial CO2 rise and climate change; here we suggest that deep water formation in the North Pacific may ...also play an important role. We present paired radiocarbon and boron isotope data from foraminifera from sediment core MD02‐2489 at 3640 m in the North East Pacific. These show a pronounced excursion during Heinrich Stadial 1, with benthic‐planktic radiocarbon offsets dropping to ~350 years, accompanied by a decrease in benthic δ11B. We suggest that this is driven by the onset of deep convection in the North Pacific, which mixes young shallow waters to depth, old deep waters to the surface, and low‐pH water from intermediate depths into the deep ocean. This deep water formation event was likely driven by an increase in surface salinity, due to subdued atmospheric/monsoonal freshwater flux during Heinrich Stadial 1. The ability of North Pacific Deep Water (NPDW) formation to explain the excursions seen in our data is demonstrated in a series of experiments with an intermediate complexity Earth system model. These experiments also show that breakdown of stratification in the North Pacific leads to a rapid ~30 ppm increase in atmospheric CO2, along with decreases in atmospheric δ13C and Δ14C, consistent with observations of the early deglaciation. Our inference of deep water formation is based mainly on results from a single sediment core, and our boron isotope data are unavoidably sparse in the key HS1 interval, so this hypothesis merits further testing. However, we note that there is independent support for breakdown of stratification in shallower waters during this period, including a minimum in δ15N, younging in intermediate water 14C, and regional warming. We also re‐evaluate deglacial changes in North Pacific productivity and carbonate preservation in light of our new data and suggest that the regional pulse of export production observed during the Bølling‐Allerød is promoted by relatively stratified conditions, with increased light availability and a shallow, potent nutricline. Overall, our work highlights the potential of NPDW formation to play a significant and hitherto unrealized role in deglacial climate change and CO2 rise.
Key Points
Deep water formed in the N Pacific during HS1 reaching 3600 mDriven by reduced atmospheric freshwater flux during stadial conditionsModel shows that N Pacific deep water can account for significant deglacial CO2 rise
Boron isotopes in marine carbonates are increasingly used to reconstruct seawater pH and atmospheric pCO2 through Earth’s history. While isotope ratio measurements from individual laboratories are ...often of high quality, it is important that records generated in different laboratories can equally be compared. Within this Boron Isotope Intercomparison Project (BIIP), we characterised the boron isotopic composition (commonly expressed in δ11B) of two marine carbonates: Geological Survey of Japan carbonate reference materials JCp‐1 (coral Porites) and JCt‐1 (giant clam Tridacna gigas). Our study has three foci: (a) to assess the extent to which oxidative pre‐treatment, aimed at removing organic material from carbonate, can influence the resulting δ11B; (b) to determine to what degree the chosen analytical approach may affect the resultant δ11B; and (c) to provide well‐constrained consensus δ11B values for JCp‐1 and JCt‐1. The resultant robust mean and associated robust standard deviation (s*) for un‐oxidised JCp‐1 is 24.36 ± 0.45‰ (2s*), compared with 24.25 ± 0.22‰ (2s*) for the same oxidised material. For un‐oxidised JCt‐1, respective compositions are 16.39 ± 0.60‰ (2s*; un‐oxidised) and 16.24 ± 0.38‰ (2s*; oxidised). The consistency between laboratories is generally better if carbonate powders were oxidatively cleaned prior to purification and measurement.
Key Points
The boron isotopic composition (δ11B) of two biogenic carbonate reference materials was measured in ten laboratories.
Unprecedentedly good interlaboratory consistency for both carbonate materials.
Gentle oxidative pre‐treatment of biogenic carbonates led to better overall consistency between laboratories.
Rationale
Organisms that grow a hard carbonate shell or skeleton, such as foraminifera, corals or molluscs, incorporate trace elements into their shell during growth that reflect the environmental ...change and biological activity they experienced during life. These geochemical signals locked within the carbonate are archives used in proxy reconstructions to study past environments and climates, to decipher taxonomy of cryptic species and to resolve evolutionary responses to climatic changes.
Methods
Here, we use laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) as a time‐resolved acquisition to quantify the elemental composition of carbonate shells and skeletons. We present the LABLASTER (Laser Ablation BLASt Through Endpoint in R) package, which imports a single time‐resolved LA‐ICP‐MS analysis, then detects when the laser has ablated through the carbonate as a function of change in signal over time and outputs key summary statistics. We provide two examples within the package: a fossil planktic foraminifer and a tropical coral skeleton.
Results
We present the first R package that automates the selection of desired data during data reduction workflows. This is achieved by automating the detection of when the laser has ablated through a sample using a smoothed time series, followed by removal of off‐target data points. The functions are flexible and adjust dynamically to maximise the duration of the desired geochemical target signal, making this package applicable to a wide range of heterogenous bioarchives. Visualisation tools for manual validation are also included.
Conclusions
LABLASTER increases transparency and repeatability by algorithmically identifying when the laser has either ablated fully through a sample or across a mineral boundary and is thus no longer documenting a geochemical signal associated with the desired sample. LABLASTER's focus on better data targeting means more accurate extraction of biological and geochemical signals.
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