Neodymium (Nd) isotope ratios are potentially valuable for paleo‐ocean circulation reconstructions because they trace water‐mass transport and mixing in the present‐day oceans. Moreover, the Atlantic ...and Pacific global end‐member Nd‐isotope values can be constrained through time, and at in‐between sites their proportions vary with past climate changes. However, an important source of uncertainty about Nd‐isotopes' applicability for paleo‐circulation studies arises from the inability to constrain past Nd‐concentrations of the end‐members. Here we address this “paleo‐Nd problem” through a Bayesian analysis. Results show that even large variability in end‐member Nd‐concentrations over the Pleistocene is unlikely to significantly impact their concentration ratio, therefore end‐member concentration changes likely have only small impacts on Nd‐isotope ratios at in‐between locations. The results support their applicability to reconstruct past Atlantic paleo‐circulation. In addition, a Nd‐isotope mass balance for Antarctic Bottom Water shows that its present‐day Nd‐isotope values are consistent with the input from its sources.
Plain Language Summary
Ocean circulation transfers heat around the Earth and characterizing its past variability is critical for understanding climate changes. Neodymium (Nd) isotopes are often used to trace past ocean circulation because they mimic deep ocean water mass mixing, and the North Atlantic and North Pacific global ocean mixing end‐member water mass compositions can be constrained through time. Therefore, temporal changes in Nd‐isotope ratios at intermediate locations like the Equatorial and South Atlantic are assumed to approximate changes in the proportions contributed by the global end‐members and thus reflect ocean circulation changes. However, Nd‐isotope ratios at intermediate locations are also sensitive to changes in Nd‐concentrations, which cannot be constrained in the global ocean end‐member water masses. This has caused the applicability of Nd‐isotope ratios for tracing paleo‐ocean circulation to be questioned. Our study examines the sensitivity of Nd‐isotope ratios at locations in‐between the global end‐members to changes in their Nd‐concentrations over recent interglacial‐glacial cycles. Results show that even substantial changes in their Nd‐concentrations likely have little impact on the Nd‐isotope ratios at intermediate locations, indicating that given the right intermediate location choices, Nd‐isotope ratios indeed reflect past deep ocean mixing, thus supporting their use to reconstruct past ocean circulation.
Key Points
Neodymium (Nd) isotopes trace modern water mass mixing, but unconstrained end‐member Nd concentrations limit their paleocirculation use
A Bayesian approach tests effects of Nd‐concentration changes in global water mass end‐members on Nd‐isotopes at intermediate locations
Results predict small impacts on Nd‐isotopes and constrain projected uncertainties in past ocean end‐member mixing reconstructions
The global ocean overturning circulation is a major means of distributing heat around the Earth, and an important trigger or amplifier of climate change. This study presents a 1.5-Myr-long neodymium ...(Nd) isotope record of Deep Sea Drilling Project Site 607, in the core of present-day North Atlantic Deep Water (NADW), the water mass that drives the overturning circulation and its Atlantic end-member (Broecker, 1991; Gordon, 1991), in order to document its composition through time. This time interval is marked by major changes in fundamental aspects of the Earth's climate, including the Mid-Pleistocene Transition (MPT) from ∼41-kyr to ∼100-kyr interglacial-glacial cycles and more intense glacials. The new record, mainly focusing on interglacial and glacial peaks, shows a pattern that mimics the record of benthic foraminiferal δ18O (Lisiecki and Raymo, 2005) in that the magnitude of interglacial-glacial Nd isotope shifts were smaller in the 41-kyr world than in the 100-kyr world. During the “900 ka event” (Clark et al., 2006), between ∼960 and 860 ka, marking the first 100-kyr interglacial-glacial cycle, the Nd isotope ratios shift abruptly to higher values, consistent with increased incursion of Southern Ocean water masses into the deep North Atlantic. This pattern was previously observed in the South Atlantic over the same time interval and interpreted as a weakened presence of NADW signal that reflected a period of disrupted deep ocean overturning circulation, termed the “MPT-AMOC crisis” (Pena and Goldstein, 2014). The Site 607 data support this interpretation and show the effects to be basin-wide. Following the disruption, an enhanced southern-sourced Nd isotope signature remained for ∼200 kyr during the period of “lukewarm interglacials” (Howe and Piotrowski, 2017; Jaccard et al., 2013), consistent with weaker overturning circulation. With the exception of this “MPT-AMOC crisis and recovery” interval, the Nd isotope ratios during interglacial peaks have been similar to present-day NADW, indicating similar interglacial North Atlantic ocean circulation dynamics both before and after the MPT. This contrasts with the pattern during glacial periods of the 100-kyr world, during which Nd isotopes have continued to follow the pattern of the 900-kyr event indicating a strong southern water mass signal, interpreted as intensified incursions of Southern Ocean water into the deep North Atlantic and consistent with a generally weaker overturning circulation during glacials.
•A new record traces the Nd isotope ratio of the deep North Atlantic over ∼1.5 Ma.•The record shows consistent interglacial values in the Early and Middle-Late Pleistocene.•The Mid-Pleistocene Transition (MPT) data indicate weak overturning circulation.•Weak overturning also characterizes the lukewarm interglacials that followed the MPT.•Intensified glacial southern water mass incursions characterize the 100-kyr world.
The causes of the Mid-Pleistocene Transition, the shift from ∼41-ky to 100-ky interglacial-glacial cycles and more intense ice ages, remain intensely debated, as this fundamental change occurred ...between ∼1,250 and 650 ka without substantial changes in astronomical climate forcings. Recent studies disagree about the relative importance of events and processes in the Northern and Southern Hemispheres, as well as whether the shift occurred gradually over several interglacial-glacial cycles or abruptly at ∼900 ka. We address these issues using a north-to-south reconstruction of the Atlantic arm of the global meridional overturning ocean circulation, a primary means for distributing heat around the globe, using neodymium (Nd) isotopes. Results reveal a period of intense erosion affecting the cratonic shields surrounding the North Atlantic between Marine Isotope Stages (MIS) 27 and 25 (∼980 and 950 ka), reflected by unusually low Nd isotope ratios in deep North Atlantic seawater. This episode preceded a major ocean circulation weakening between MIS 25 and 21 (950 and 860 ka) that coincided with the first ∼100-ky-long interglacial-glacial onset of Northern Hemisphere glaciation at around 2.4 to 2.8 Ma. The data point to a Northern Hemisphere
sourced initiation for the transition, possibly induced through regolith loss and increased exposure of the crystalline bedrock, which would lead to increased friction, enabling larger ice sheets that are characteristic of the 100-ky interglacial-glacial cycles.
Most of the fossil corals in the elevated reef terraces along the Gulf of Aqaba (GOA) were extensively altered to calcite. This observation indicates extensive interaction with freshwater, possibly ...when the terraces passed through a coastal aquifer that existed along the shores of the GOA, implying a wetter climate during the time of recrystallization from aragonite to calcite. Thus, dating of the recrystallization events should yield the timing of past wetter conditions in the current hyper-arid area of the GOA. In the present study, 18 aragonite and calcite corals were collected from several elevated coral reef terraces off the coast, south of the city of Aqaba. While aragonite corals were dated with the conventional closed system age equation (assuming zero initial Th), the dating of the calcite corals required the development of adequate equations to allow the calculation of both the initial formation age of the aragonite corals and the time of recrystallization to calcite. The two age calculations were based on the assumptions that each reef terrace went through a single and rapid recrystallization event and that the pristine aragonite corals were characterized by a rather uniform initial U concentration, typical for pristine modern corals. Two recrystallization events were identified at 104±6ka and 124±8ka. The ages coincide with the timing of sapropel events S4 and S5, respectively, when the African monsoon induced enhanced wetness in the desert area. Considering the age uncertainties, the times of formation of the two major reef terraces are estimated to be ∼124ka (reef terrace R2) and ∼130ka (reef terrace R3), matching the peaks in the global sea level during the last interglacial MIS 5e stage. Apparently, sea level of the GOA did not fluctuate a lot during the period between ∼130ka and ∼104ka and remained close to the Marine Isotopic stage (MIS) 5e highstand. The availability of freshwater (during the sapropel periods) and limited sea level fluctuations facilitated the recrystallization of the GOA reef corals to calcite.
In recent decades geochemical-paleoclimatic data allowed for the discovery of the persistent past occurrence of abrupt changes and transitions within the climate system. This highlights the need to ...develop a detailed understanding of the dynamics by which the various components of the climate system interact. Using a range of geochemical tools applied to marine sediments from the Atlantic Ocean, this dissertation explores a number of paleoclimatic transitions, in the context of two major climatic components: ocean circulation and dust supply. The first chapter focuses on the onset of the shift from ~41-to-100-kyr interglacial-glacial cyclicity between ~1250-700 ka (also termed ‘the mid-Pleistocene transition’ or MPT), which is a subject of intense controversy, as this fundamental change occurred without substantial changes in the astronomical climate forcings (e.g.: Clark et al., 2006) and the timing, locations, and identities of its drivers remain unresolved. Recent studies disagree whether the transition occurred gradually over several interglacial-glacial cycles (Ford et al., 2016) or abruptly at around 900 ka (Elderfield et al., 2012), as well as whether it was driven by events in the Northern (Detlef et al., 2018; Ford et al., 2016; Kender et al., 2018; Pena and Goldstein, 2014; Sosdian and Rosenthal, 2009) or the Southern (Elderfield et al., 2012; Farmer et al., 2019; Hasenfratz et al., 2019; Lear et al., 2016) Hemisphere. In chapter one, we address these questions using a new north-to-south reconstruction, based on sedimentary neodymium isotope ratios, of the Atlantic meridional overturning circulation (AMOC; Broecker, 1991), a primary means for distributing heat around the globe. Our results reveal an abrupt glacial erosional event in the cratonic shields surrounding the North Atlantic and a breakdown of the ice-sheet at ~965-950 ka (Marine Isotope Stage or MIS-26 (Lisiecki and Raymo, 2005)). This event directly preceded a major weakening of the ocean conveyor circulation (Pena and Goldstein, 2014), which our Atlantic transect shows had global impact, and which coincided with the first ~100-kyr-long interglacial-glacial cycle. Moreover, between ~1250-965 ka, leading up to the ‘MIS-26 erosional event’, increasing cratonic weathering contributions to the deep North Atlantic during glacial periods point to increasingly effective continental ice-sheet erosional activity over this critical time-interval. The evidence thus implicates both long-term ice-sheet processes and an abrupt event that occurred just prior to the cyclicity shift. This new global view reveals evidence of a Northern Hemisphere-sourced initiation for the MPT, induced through regolith loss (Clark et al., 2006), leading to more stable ice-sheets facilitated by increased friction. These processes enabled enhanced ice sheet growth and thickening characteristic of the 100-kyr interglacial-glacial cycles. In the second chapter we further explore the changes in the AMOC, which is widely believed to weaken during glacials and strengthen during interglacials, and expand the temporal scale of Nd-isotopes profiles, presenting data from the North to the Equatorial Atlantic over the past ~1.5 Ma. We have identified five modes of the AMOC. The most common ones are (1) the “interglacial norm”, where the Northern Sourced Water (NSW) signal remains strong into the South Atlantic, and (2) the “glacial norm”, indicating a weaker AMOC, with southern source water (SSW) extending into the deep North Atlantic. Less common are the (3) “weak AMOC” mode, typical of Heinrich events, the AMOC-crisis event (MIS 24-22) during the MPT, and MIS 10 and 16, where even the deep North Atlantic shows a strong SSW signal, and its counterpart the (4) “ultra-strong AMOC”, in MIS 9, 11, 19, 21 and 25, when the NSW signal is unusually strong south of the equator, similar to the present-day. (5) “AMOC-Divergence” (MISs 41, 27, 26 and 14), possibly indicating a decoupling between the northern and southern AMOC cells. The AMOC time-slice profiles provide a useful new framework to relate climate past changes directly to concurrent ocean circulation through time. The third chapter reports a first attempt to address changes in dust deposition due to the formation of the Sahara Desert, as documented at the nutrient-poor region, of the Great-Bahama-Bank carbonate platform. The dust deposition records produced here span over ~14 million years, thus providing a direct, coarse resolution examination, of the co-evolution of the two systems, the Great-Bahama-Bank and the Saharan Desert dust source, since the mid-Miocene till the late Pleistocene. Here, I utilized a suite of newly applied methods across the hypothesized time of Sahara Desert formation on sediments from two core sites at the Great Bahama Bank. Results from the Great Bahama Bank marine sediments show changes in the accumulation of terrigenous material through time with a possible notable increase occurring around ~11 Ma and ~4 Ma. The geochemical evidences point to at least two possible sources: Northwest Africa (namely, the northern Sahel region) and the northern part of South America (namely, the Orinoco drainage basin), which is more evident in samples younger than 4 Ma. The emergence of the northern-South-American isotopic signature suggests that the closure of the Panama Isthmus may not only have played a crucial role in the evolution of the Great-Bahama-Bank, as suggested in earlier studies (Reijmer et al., 2002), but also led to the transport of weathering products all the way to the northern part of the Gulf of Mexico. Taken together, these first results highlight the potential of the Great Bahama Bank sediments as archives of Saharan dust deposition and global changes.
The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB‐δ15N) using the oxidation–denitrifier (O–D) method provides a novel tool to study ...past changes in N cycling processes.
Methods
We report a set of methodological improvements to the O–D method, including (a) a method for sealing the reaction vials in which the oxidation of organic N to NO3− takes place, (b) a recipe for bypassing the pH adjustment step before the bacterial conversion of NO3− to N2O, and (c) a method for storing recrystallized dipotassium peroxodisulfate (K2S2O8) under Ar atmosphere.
Results
The new sealing method eliminates the occasional contamination and vial breakage that occurred previously while increasing sample throughput. The protocol for bypassing pH adjustment does not affect BB‐δ15N, and it significantly reduces the processing time. Storage of K2S2O8 reagent under Ar atmosphere produces stable oxidation blanks over more than 3.5 years. We report analytical blanks, accuracy, and precision for this methodology from eight users over the course of ~3.5 years of analyses at the Max Planck Institute for Chemistry. Our method produces analytical blanks characterized by low N content (0.30 ± 0.13 nmol N, 1σ, n = 195) and stable δ15N (−2.20 ± 3.13‰, n = 195). The analysis of reference amino acid standards USGS 40 and USGS 65 indicates an overall accuracy of −0.23 ± 0.35‰ (1σ, n = 891). The analysis of in‐house fossil standards gives similar analytical precision (1σ) across a range of BB‐δ15N values and biominerals: zooxanthellate coral standard PO‐1 (6.08 ± 0.21‰, n = 267), azooxanthellate coral standard LO‐1 (10.20 ± 0.28‰, n = 258), foraminifera standard MF‐1 (5.92 ± 0.28‰, n = 243), and tooth enamel AG‐Lox (4.06 ± 0.49‰, n = 78).
Conclusions
The methodological improvements significantly increase sample throughput without compromising analytical precision or accuracy down to 1 nmol of N.
The global thermohaline circulation plays a major role in regulating global climate, driven by the formation of North Atlantic Deep Water (NADW). ODP Site 1063 on the Bermuda Rise, at the interface ...of NADW and Southern Ocean‐sourced water, appears an ideal location to study the relationships between ocean circulation and climate. This study reports Nd isotope ratios at Site 1063 that extend to ~1 Ma. The data show Nd isotope values during portions of interglacials that are much lower than modern NADW. However, interglacial Nd isotope values at Site 607, located within the core of NADW, off the abyssal seafloor in the North Atlantic, are consistently similar to modern NADW. In contrast to glacial values, we infer that interglacial Nd isotopes at Site 1063 are not representative of NADW and do not solely record water mass mixing. We conclude that the low Ndisotope ratios reflect regional particle‐seawater exchange as a consequence of input of freshly ground bedrock from the Canadian shield, which is eroded into the North Atlantic during major ice sheet retreats. The result is a deep, thin, and regionally constrained layer of seawater tagged with this anomalous low Nd isotope signature that is unrepresentative of the Atlantic Meridional Overturning Circulation (AMOC). We suggest that a benthic nepheloid layer, whose development is linked to the behavior of a deep‐recirculating gyre system, regulated by the interaction between the Gulf Stream and the deep western boundary current, facilitates the periodic masking of the Nd isotope signature of the North Atlantic AMOC end‐member in this region at these depths.
Key Points
Glacial‐interglacial changes at the deep Bermuda Rise during the Middle and early Lower Pleistocene were recorded with εNd
Glacial εNd values in the deep Bermuda Rise record the AMOC, interglacial εNd values do not
Dynamic benthic nepheloid layers temporally mask εNd at the deep Bermuda Rise during parts of interglacials
Early onset preeclampsia and cerebral palsy: a double hit model? Mor, Omer, BMSc; Stavsky, Moshe, BMSc; Yitshak-Sade, Maayan, MPH ...
American journal of obstetrics and gynecology,
2016, January 2016, 2016-Jan, 2016-01-00, 20160101, Letnik:
214, Številka:
1
Journal Article
Recenzirano
Background Cerebral palsy (CP) is a late sequel of pregnancy, and the role of preeclampsia is debatable. Objective The aims of this study were to determine the association between preeclampsia and ...cerebral palsy and to determine the risk factors for the development of cerebral palsy in these patients. Study Design A retrospective population-based cohort study was designed that included 229,192 singleton pregnancies. The study population was divided into 2 groups: (1) patients with preeclampsia (n = 9749) and (2) normotensive gestations (n = 219,443). Generalized Estimating Equation multiple logistic regression models were performed to study the associations among preeclampsia, small for gestational age, gestational age at delivery, and the risk factors for the development of cerebral palsy in neonates of women with preeclampsia. Results The rate of cerebral palsy was double in patients with preeclampsia than in the normotensive group (0.2% vs 0.1%; P = .015); early onset preeclampsia and small for gestational age were independent risk factors for the subsequent development of cerebral palsy (odds ratio, 8.639 95% confidence interval, 4.269–17.480; odds ratio, 2.737 95% confidence interval, 1.937–3.868, respectively). A second model was conducted to determine the risk factors for the development of cerebral palsy in women with preeclampsia. Birth asphyxia, complications of prematurity, and neonatal infectious morbidity, but not small for gestational age or gestational age at delivery, were independent risk factors for the development of cerebral palsy. Conclusion In a comparison with normal pregnant women, the rate of cerebral palsy is double among patients with preeclampsia, especially those with early-onset disease. Early-onset preeclampsia is an independent risk factor for cerebral palsy. Among women with preeclampsia, the presence of neonatal infectious morbidity, birth asphyxia, and complications of prematurity are independent risk factors for the development of cerebral palsy, which further supports the role of a multi-hit model in the pathogenesis of this syndrome.
Objectives: To analyze in a retrospective cohort study the outcomes of pregnancies with isolated oligohydramnios at the late preterm period (34-36.6 weeks of gestation).
Study design: This ...retrospective cohort study included three groups of women: (1) Women with isolated oligohydramnios whose pregnancy was managed conservatively (n = 33 births); (2) women with isolated oligohydramnios who were managed actively (i.e. induction of labor) (n = 111 births); and (3) a control group including women with normal amount of amniotic fluid who had a spontaneous late preterm delivery (n = 10,445 births). Maternal and fetal characteristics and obstetrics outcomes were collected from a computerized database of all deliveries at Soroka University Medical Center during the study period.
Results: Our cohort included 10,589 births. The rate of inducing labor was higher in the oligohydramnios groups compared to the controls (p < .001). There was an increase in the rate of cesarean section (CS) in the conservative treatment group (p < .001), compared with the other groups. Conservative management was associated with higher rates of maternal infection (p = .026), chorioamnionitis (p = .01), and transitory tachypnea of the newborn (p = .02). After controlling for confounding factors, mal presentation (OR = 19.9), and a prior CS (OR = 2.4) were independently associated with an increased risk for CS, while induction of labor was associated with a reduced risk for CS (OR = 0.28).
Conclusions: Women with late preterm isolated oligohydramnios had a higher rate of induction of labor than women with a normal amount of amniotic fluid. Induction of labor seems to be beneficial to both the neonate and the mother as seen by a lower rate of CS conducted in this group, as well as lower maternal and neonatal morbidity in comparison to the conservative group. Therefore, women with oligohydramnios at late preterm may benefit from induction of labor.
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