Transect Of Cores Research Articles

Volcanism and climate change as drivers in Holocene depositional dynamic of Laguna del Maule (Andes of central Chile – 36° S)

Abstract. Late Quaternary volcanic basins are active landscapes from which detailed archives of past climate and seismic and volcanic activity can be obtained. A multidisciplinary study performed on a transect of sediment cores was used to reconstruct the depositional evolution of the high-elevation Laguna del Maule (LdM) (36∘ S, 2180 m a.s.l., Chilean Andes). The recovered 5 m composite sediment sequence includes two thick turbidite units (LT1 and LT2) and numerous tephra layers (23 ash and 6 lapilli). We produced an age model based on nine new 14C AMS dates, existing 210Pb and 137Cs data, and the Quizapú ash horizon (1932 CE). According to this age model, the relatively drier Early Holocene was followed by a phase of increased productivity during the mid-Holocene and higher lake levels after 4.0 ka cal BP. Major hydroclimate transitions occurred at ca. 11, 8.0, 4.0 and 0.5 ka cal BP. Decreased summer insolation and winter precipitation due to a southward shift in the southern westerly winds and a strengthened Pacific Subtropical High could explain Early Holocene lower lake levels. Increased biological productivity during the mid-Holocene (∼8.0 to 6.0 ka cal BP) is coeval with a warm–dry phase described for much of southern South America. Periods of higher lake productivity are synchronous to a higher frequency of volcanic events. During the Late Holocene, the tephra layers show compositional changes suggesting a transition from silica-rich to silica-poor magmas at around 4.0 ka cal BP. This transition was synchronous with increased variability of sedimentary facies and geochemical proxies, indicating higher lake levels and increased moisture at LdM after 4.0 ka cal BP, most likely caused by the inception of current El Niño–Southern Oscillation and Pacific Decadal Oscillation (ENSO–PDO) dynamics in central Chile.

A new model of Holocene reef initiation and growth in response to sea-level rise on the Southern Great Barrier Reef

The fossil record provides valuable data for improving our understanding of both past and future reef resilience and vulnerability to environmental change. The spatial and temporal pattern of the initiation of the Holocene Great Barrier Reef presents a case study of reef response to rapid sea-level rise. Past studies have been limited by the lack of well-dated and closely spaced reef core transects and have not closely examined the composition of the reef-building communities through time. This study presents 80 new high precision UTh and 5 radiocarbon ages from twelve new cores located along three transects across different geomorphic and hydrodynamic settings of One Tree Reef, southern Great Barrier Reef, to document three distinct stages of Holocene reef development in unprecedented detail. Temporal constraints on changing paleoecological assemblages of coral, coralline algae and associated biota revealed three distinct phases of reef development, consisting of: 1)a fast, shallow and clear-water reef initiation from 8.2 until 8 ka; 2)a shift to slower, deeper and more turbid-water reef growth from 8 to 7 ka; and 3)a return to shallow and rapid branching coral growth in clear-water conditions as the reef “catches up” to sea-level. A minimum lag prior to reef initiation of 700 years was identified, which differs in length depending on reef environment and Pleistocene substrate height. In this new model, reef growth initiated on the topographically lower leeward margin and patch reef, prior to the start of windward margin development, contrary to the traditional reef growth model. While there was a shift to conditions less favorable for reef growth at 8 ka, this did not prevent the slow accretion of more sediment-tolerant coral communities. The majority of the reef reached sea level by ~6 ka. This new conceptual model of Holocene reef growth provides new constraints on changes in paleoenvironment that controlled reef community composition and growth trajectories through sea-level rise following inundation.

Ramp reef depositional facies model for the Mid-Pliocene Golden Gates Reef Member of the Tamiami Formation, South Florida

The depositional setting for northern most Atlantic coral reef development during the Mid-Pliocene Warm Period is a gentle sloping mixed carbonate–siliciclastic ramp. Five core transects document the distribution of the reef complex in an area approximately 65 by 12 km, and nine repetitive depositional facies are identified. Paleoecological and sedimentological evidence documents facies development along a water depth–energy gradient. The mid-Pliocene Tamiami Formation is characterized by coral boundstone developed over level skeletal rudstone depositional units dominated by mollusks. Hyotissa haitensis (Sowerby), one of the last Gryphaeid oysters, is the dominant fossil found in the most continuous skeletal facies and overlies deeper water green clay facies, the only facies with pelagic foraminifera. Ground penetrating radar documents reef depositional topography, onlapping stratigraphy and two episodes of reef growth. Two cycles of deposition are recognized, separated by subaerial exposure. The coral boundstone and the skeletal rudstone exhibit both high primary and secondary porosity and overlie the impermeable clay facies. The upper surface of the coral boundstone lies at ~ 4 m in elevation whereas contemporaneous estuarine deposits are found to the north at elevations of 20–25 m. High porosity bank reef complexes along a shallow dipping ramp provide an alternative to the standard model of reef and porosity development along the outer shelf margin. Understanding the differences in associated facies between these two depositional environments permits better interpretation of observed heterogeneities in subsurface geobodies associated with inner shelf and platform settings.

Floodplain chronology and sedimentation rates for the past 200 years derived from trace element gradients, organic compounds, and numerical modeling

This study evaluates the applicability of trace element and organic contaminant data from a floodplain cross-section as the basis for a numerical model of spatial floodplain dynamics. Using threshold values of pollution-sensitive trace elements and market introduction dates of organic xenobiotics, the sampled sediment is assigned to historical phases to develop a sediment chronology. The investigation is based on a 60-m wide core transect from which sediment samples were analyzed to determine grain-size distribution, trace element inventory, and organic xenobiotic content. In addition, floodplain inundation, flow velocities, and the amount of sediment deposited were numerically modeled using Delft3D to verify the analyses results; conversely, the results of the sedimentary analysis served the input data for the model. Changes in floodplain morphology were interpreted on the basis of a digital elevation model (1 m resolution), historical maps from 1865 AD, and field surveys. The architecture of the alluvial sediments was examined in the cores accounting recent floodplain relief and possible historical factors. The results show a broad range of heavy metal pollutants and the presence of 57 volatile organic compounds in a pattern that reflects multiple deposition processes and phases. Based on these results and the model verification, the sediments were assigned to pre-industrial, industrial, and post-industrial phases, and sedimentation rates of 0.6–1.3 cm a−1 were estimated. The results of this study contribute to a better understanding of the development of small meandering gravel-bed rivers with large floodplains, where suspended sediments predominate.

Southwest Pacific Vertical Structure Influences on Oceanic Carbon Storage Since the Last Glacial Maximum

AbstractLower atmospheric CO2 concentrations during the Last Glacial Maximum (LGM; 23.0–18.0 ka) have been attributed to the sequestration of respired carbon in the ocean interior, yet the mechanism responsible for the release of this CO2 during the deglaciation remains uncertain. Here we present calculations of vertical differences in oxygen and carbon isotopes (∆δ18O and ∆δ13C, respectively) from a depth transect of southwest Pacific Ocean sediment cores to reconstruct changes in water mass structure and CO2 storage. During the Last Glacial Maximum, ∆δ18O indicates a more homogenous deep Pacific below 1,100 m, whereas regional ∆δ13C elucidates greater sequestration of CO2 in two distinct layers: enhanced CO2 storage at intermediate depths between ~940 and 1,400 m, and significantly more CO2 at 1,600 m and below. This highlights an isolated glacial intermediate water mass and places the main geochemical divide at least 500 m shallower than the Holocene. During the initial stages of the deglaciation in Heinrich Stadial 1 (17.5–14.5 ka), restructuring of the upper ~2,000 m of the southwest Pacific water column coincided with sea‐ice retreat and rapid CO2 release from intermediate depths, while CO2 release from the deep ocean was earlier and more gradual than waters above it. These changes suggest that sea‐ice retreat and shifts in Southern Ocean frontal locations contributed to rapid CO2 ventilation from the Southern Ocean's intermediate depths and gradual ventilation from the deep ocean during the early deglaciation.

Modern, Preindustrial, and Past (Last 25 ka) Carbon Isotopic (δ13C) Variability in the Surface Waters of the Southwest Pacific

AbstractCarbon stable isotopes (δ13C) in modern seawater samples and planktic foraminifera Globigerina bulloides from core top and downcore sediments are used to estimate the distribution of δ13C of dissolved inorganic carbon (DIC) in the surface waters of the southwest Pacific in the modern, preindustrial (PI), and over the last 25 kyr. The predicted δ13C distribution in the modern (δ13CDIC), PI (δ13CPI), and late Holocene (from planktic foraminifera Globigerina bulloides [temperature corrected δ13CG.bulloidesTC] from core tops) displays a broad peak at the subtropical front) and subantarctic surface waters due to the combination of high biological productivity and thermodynamic air‐sea gas exchange of CO2 in this region. The estimated δ13CPI values and measured δ13CG.bulloidesTC values from the core tops are higher than the modern values due to the Suess Effect. However, there is poor agreement between the δ13CPI values and core top δ13CG.bulloidesTC values south of 40°S as the back‐calculation approach using chlorofluorocarbon‐11 (CFC‐11) method for removing the anthropogenic δ13C is not effective at these higher southern latitudes. The δ13CG.bulloidesTC from a latitudinal transect of cores in the southwest Pacific were compiled by region using a Monte Carlo approach to determine the long‐term trends in δ13C over the last 25 kyr. Glacial subantarctic δ13CG.bulloidesTC values are low, while subtropical δ13C G.bulloidesTC are high. The peak in δ13CG.bulloidesTC values shifts south in the early Holocene. These latitudinal variations in δ13C G.bulloidesTC are linked to changes in ocean circulation, biological productivity (associated with the shifts in the subtropical front), and air‐sea CO2 exchange, likely related to the structure and position of the Southern Hemisphere Westerly Wind in the South Pacific region.

Lessons from a modern carbonate sandbody – A personal experience of comparative sedimentology

AbstractThe notion that examination of modern environments can help one better decipher ancient deposits does not seem so unusual, but this view was incrementally developed through decades of studies that have shown just that. Presented here is a brief example of comparative sedimentology with emphasis placed on the lessons learned from the study of a modern ooid sandbody that heightens one's ability to describe and interpret analogous subsurface settings. Core transects and surface sediment mapping in the Joulters sandbody of Great Bahama Bank illustrate the three‐dimensional characteristics of an upward coarsening and shallowing cycle and highlight the internal complexity of a sandbody through time and space. This modern example graphically highlights difficulties in interpretation and correlation of grainstone cycles in analogous subsurface platform carbonate reservoirs.

Tracking Holocene drift-ice limits on the northwest–southwest Iceland shelf: Comparing proxy data with observation and historical evidence

ABSTRACTWe detail variations in the weight percent (wt%) of quartz, a proxy for drift ice, in fifteen marine sediment cores from the northwest, west, and southwest Iceland shelf throughout the past 10 cal ka BP. We present the first map of iceberg distributions in Iceland waters between 1983 and 2011 and a new compilation of sea-ice records in the century from 850 to 1950 CE. The wt% of quartz, determined by quantitative X-ray diffraction (qXRD) analysis, is used to evaluate changes in the importation of drift ice. Small wt% of quartz were added to milled basalt (0% quartz), and to a mixture of non-clay and clay minerals; the qXRD method replicated 0 percent quartz, while measured 1–3 percent quartz always resulted in a “presence” estimate. The outer sites in the northwest sector lie close to the average position of the sea-ice margin between 1870 and 1920 CE; the southwest shelf sites lie south of this limit. Transects of cores along the Húnaflói and Djúpáll troughs indicate that the traces of drift ice decrease rapidly landward from the outer sites. The cores from the west/southwest of Iceland have limited amounts of quartz, generally possibly limited incursions of drift ice.

Evidence for Shelf Acidification During the Onset of the Paleocene‐Eocene Thermal Maximum

AbstractA transect of paleoshelf cores from Maryland and New Jersey contains an ~0.19‐ to 1.61‐m‐thick interval with reduced percentages of carbonate during the onset of the Paleocene‐Eocene Thermal Maximum (PETM). Outer paleoshelf cores are barren of nannofossils and correspond to two minor disconformities. Middle paleoshelf cores contain a mixture of samples devoid of nannofossils and those with rare specimens characterized by significant dissolution (i.e., etching). The magnitude of the decrease in carbonate cannot be explained by dilution by clastic material or dissolution resulting from the oxidation of organic matter during early diagenesis. The observed preservation pattern implies a shoaling of the calcite compensation depth and lysocline to the middle shelf. This reduced carbonate interval is observed during the onset of the PETM on other continental margins raising the possibility that extreme shoaling of the calcite compensation depth and lysocline was a global signal, which is more significant than in previous estimates for the PETM. An alternative scenario is that shoaling was restricted to the northwest Atlantic, enhanced by regional and local factors (eutrophication from rivers and microbial activity associated with warming) that exacerbated the impact of acidification on the shelf.

Active near-surface growth faulting and late Holocene history of motion: Matagorda peninsula, Texas

The structural framework of the northern Gulf of Mexico coastal zone includes numerous growth fault systems. Neotectonic processes in coastal environments here have been shown to be important contributors to subsidence and relative sea-level rise, as well as having significant influences on sedimentary accretion processes. One active growth fault at East Matagorda Peninsula, Texas was subjected to detailed study to address the hypotheses that (1) this fault was present and active before the peak of regional hydrocarbon recovery in the mid-20th century, and that (2) slip along this fault has been episodic over the resolved period of record. To characterize the late Holocene behavior of this fault, three core transects were established normal to the surface fault trace, and stratigraphic analysis was conducted with the use of radiocarbon (14C) for chronological control. Correlation of time-equivalent stratigraphic boundaries reveals a maximum total late Holocene offset of 0.7 m. The Matagorda fault was in place long before the peak of regional oil and gas extraction, but experienced a late Holocene rejuvenation, initiated between 2500 and 1500 years BP. The kinematic behavior of the fault expressed as temporally and spatially variable measures of expanded stratigraphic thicknesses on the downthrown extent suggest that sediment loading from the transgressive Matagorda barrier complex may have contributed to this rejuvenation, and that slip along the fault persists to the present. Radiocarbon chronology also revealed clear differences in the magnitude of fault throw during the late Holocene, with maximum displacement (0.7 m) occurring at the southwestern-most transect and minimum throw at the northeastern-most transect (0.2 m).

Hemipelagic and turbiditic deposits constrain lower Bengal Fan depositional history through Pleistocene climate, monsoon, and sea level transitions

The Bengal Fan contains the most complete record of Himalayan climate and tectonics. As the largest fan system of the world it is fed by the Ganges-Brahmaputra river systems. International Ocean Discovery Program (IODP) Expedition 354 cored seven sites along an east-west core transect at 8°N in the lower Bengal Fan. We studied the Pleistocene sections of these sites to provide constraints on the depositional history of the last 1.25 Ma. Low-resolution bio- and magnetostratigraphic constraints identified two regionally-extensive hemipelagic units and intercalated turbidite deposits that provide a chronostratigraphic framework for the 320 km transect. Using a combination of lithostratigraphy and sediment physical properties that vary on orbital timescales, we tuned high-resolution data sets to the LR04 benthic δ18O isotope stack to obtain new constraints on the timing of depositional changes.Above the underlying Unit 1, which consists of Early to Middle Pleistocene turbiditic deposits, we divide four more units for the last 1.25 Ma: Unit 2, a Middle Pleistocene hemipelagic layer deposited at all sites from Marine Isotopic Stages (MIS) 37 to ∼17 (∼1.24–0.68 Ma) during the entire time of the Mid-Pleistocene Transition, when the Bengal Fan depocenter must have been distal to our transect; Unit 3, mostly turbidites with some intercalated hemipelgic sediments deposited from ∼MIS 16–∼13 (∼0.68–0.48 Ma), with sandy lithologies only found at the easternmost site on the west flank of the Ninetyeast Ridge; Unit 4, massive turbiditic sediments that started to dominate deposition with the Mid-Brunhes Transition (∼MIS 12) until MIS 7/8 (∼0.48–0.25 Ma), when the channel-levee system was focused east of the 85°E basement ridge at 8°N; and Unit 5, a Late Pleistocene hemipelagic layer at the top that spans MIS 7/8–1 (∼0.25 Ma to present) while the turbidite deposition was focused west of the 85°E basement ridge at 8°N. We find evidence that, on these timescales, deposition across the lower Bengal Fan changed with the evolution of Pleistocene climate, monsoon and sea-level history and hence responded to external controls on sediment accumulation and fan architecture rather than only fan-internal, autocyclic mechanisms.

Seismic and lithofacies characterization of a gravity core transect down the submarine Tuaheni Landslide Complex, NE New Zealand

Abstract The southern Tuaheni Landslide Complex (TLC) at the Hikurangi subduction margin displays distinctive morphological features along its distribution over the Tuaheni slope offshore Gisborne, New Zealand. We here present first analyses of a gravity core transect that systematically samples surficial sediments from the source area to the toe of this landslide complex, thus providing important new insight into shallow lithological variation in the slide complex. Geophysical and geochemical core logs and core descriptions form the basis for a characterization of representative sediment successions that are indicative of the respective slope segment of recovery. Our results show that the lithology of surficial sediments varies significantly along the length of the landslide complex. Depending on the slope segment observed, this variation includes post-Last Glacial Maximum (LGM) outer-shelf sediments, and hemipelagic drape and near-surface reworked debris avalanche deposits, as well as multiple intercalated thinner turbidites and tephra layers at the distal end of the profile. Lithological downslope variability suggests ongoing mass transport events through the late Holocene that were likely to have been limited to small mud-turbidite flows. Integration with acoustic sub-bottom imagery reveals the presence of multiple stacked mass-transport deposits at depth, contrasting with previous interpretations of a single parent failure.

A critical evaluation of coral Ba/Ca, Mn/Ca and Y/Ca ratios as indicators of terrestrial input: New data from the Great Barrier Reef, Australia

Coral trace element proxies of terrestrial runoff are widely applied to document and quantify historical changes in river discharge, constituent loads and land disturbance. However, some studies show poor replication between trace element records where cores from multiple coral colonies have been analyzed. Conflicting interpretations also exist on the environmental variable the coral trace element proxy is recording. Indeed, few studies have examined trace element behavior in the estuarine mixing zone that influence the coral records and only limited data on river discharge and constituent loads are available to validate the proxy records of terrestrial runoff. This study examined the behavior of Barium (Ba), Manganese (Mn) and Yttrium (Y) in the Burdekin River estuarine mixing zone, north-eastern Australia during several flood events and investigated the ability of coral Ba/Ca, Mn/Ca and Y/Ca ratios to record the variability of measured Burdekin River discharge and suspended sediment loads along a transect of replicate coral cores (10 cores from 5 locations). The results show limited evidence for Ba desorption from suspended sediments in the estuarine mixing zone while considerable desorption of Mn over a wide salinity mixing gradient was evident. Y showed evidence of removal in the initial mixing zone of the estuary (0–5 PSU) but displayed a relatively flat pattern thereafter indicating that it was not diluted by seawater mixing. The coral trace element records generally showed poor agreement within sites and the Ba/Ca ratios in some corals did not respond to regional river discharge, although coral luminescent lines were present in all cores and were significantly correlated with each other and Burdekin River discharge. While it was difficult to disentangle the influence of Burdekin River discharge and suspended sediment loads on the coral Ba/Ca proxy, several lines of evidence point towards discharge as the major contributor to this proxy in this region. A combination of factors likely influence the coral Ba/Ca ratio including an as yet undetermined physiological mechanism possibly related to crystallography or species, distance of the coral to the river mouth, variability in river discharge (including both annual totals and peaks, and local influences), variability in sediment load, the behavior of trace elements in the estuarine mixing zone, the hydrodynamics of flood plumes in the marine environment and the water depth of the coral. While the coral Mn/Ca and Y/Ca ratios did not respond to Burdekin River discharge events, the considerable desorption of Mn and the elevated Y throughout the estuarine mixing zone shows some promise in their ability to record long-term changes in suspended sediment loading and large terrestrial disturbances. Future studies examining coral records of terrestrial runoff should examine trace element behavior in the river estuarine mixing zone of influence, establish a correlation between the trace element and river discharge (such as luminescent lines) and demonstrate replication between different coral colonies.

Formation and evolution of back‐barrier perched lakes in rocky coasts: An example of a Holocene system in north‐west Spain

AbstractCoastal back‐barrier perched lakes are freshwater bodies that are elevated over sea‐level and are not directly subjected to the inflow of seawater. This study provides a detailed reconstruction of the Doniños back‐barrier perched lake that developed at the end of a small river valley in the rocky coast of the north‐west Iberian Peninsula during the Holocene transgression. Its sequence stratigraphy was reconstructed based on a core transect across the system, the analyses of its lithofacies and microfossil assemblages, and a high‐resolution radiocarbon‐based chronology. The Doniños perched lake was formed ca 4·5 ka bp. The setting of the perched lake was favoured by Late Holocene sea‐level stabilization and the formation of a barrier and back‐barrier basin, which was contemporaneous with the high systems tract period. This basin developed over marine and lagoonal sediments deposited between 10·2 ka bp and 8·0 ka bp, during rapidly rising sea‐level characteristic of the transgressive systems track period. At 1·1 ka bp, the barrier was breached and the perched lake was partially emptied, causing the erosion of the back‐barrier basin sediments and a significant sedimentary hiatus. Both enhanced storminess and human intervention were likely to be responsible for this event. After 1 ka bp, the barrier reclosed and the present‐day lake was reformed, with the water level reaching as high as 5 m above mean sea‐level. The depositional evolution of the Doniños system serves as a model of coastal back‐barrier perched lakes in coastal clastic systems that have developed over gently seaward‐dipping rugged substrates at small distances from the shoreline and under conditions of rising sea‐level and high sediment supply. A review of estuaries, back‐barrier lagoons, pocket beaches and back‐barrier perched lakes in the rocky coast of north‐west Spain shows that the elevation of the bedrock is the main factor controlling the origin and evolution of these systems.

How Could a Freshwater Swamp Produce a Chemical Signature Characteristic of a Saltmarsh?

Reduction–oxidation (redox) reaction conditions, which are of great importance for the soil chemistry of coastal marshes, can be temporally dynamic. We present a transect of cores from northwest Florida wherein radical postdepositional changes in the redox regime has created atypical geochemical profiles at the bottom of the sedimentary column. The stratigraphy is consistent along the transect, consisting of, from the bottom upward, carbonate bedrock, a gray clay, an organic mud section, a dense clay layer, and an upper organic mud unit representing the current saltwater marsh. However, the geochemical signature of the lower organic mud unit suggests pervasive redox reactions, although the interval has been identified as representing a freshwater marsh, an unlikely environment for such conditions. Analyses indicate that this discrepancy results from postdepositional diagenesis driven by millennial-scale environmental parameters. Rising sea level that led to the deposition of the capping clay layer, create...

On the influence of post-LGM ice shelf loss and grounding zone sedimentation on West Antarctic ice sheet stability

The Whales Deep Basin in eastern Ross Sea was occupied by the paleo-Bindschadler Ice Stream during the Last Glacial Maximum (LGM). Previous studies showed that megascale glacial lineations (MSGLs) extend to the continental shelf edge. In a landward direction, these lineations are buried by an overlapping stack of backstepped grounding zone wedges (GZWs). The constraints require that the West Antarctic Ice Sheet (WAIS) grounded at least seven times within 60km of the shelf edge but the morphological relationships in the Whales Deep Basin cannot be used to uniquely reveal the magnitudes of either grounding line and/or calving front retreat and re-advance between the successive groundings. Here we show a new regional transect of cores from the basin which demonstrates that there were no major back and forth oscillations of either the grounding line or calving front during the overall retreat of grounded and floating ice. Instead, the new sedimentologic data show that a small ice shelf formed as the grounding line retreated from the shelf edge and backstepped over the outer continental shelf. An event bed records breakup of the ice shelf that covered the outer shelf. The stratal relationships indicate that the ice-shelf collapse occurred during the fourth grounding and that the grounding line remained on the outer continental shelf for three additional groundings. We infer that accelerated ice stream flow (due to loss of buttressing) increased sediment flux to the grounding line and thinned the WAIS at the grounding line. Following the seventh grounding, thinning contributed to an abrupt 200km retreat of the grounding line to the middle continental shelf and a large ice shelf re-formed over the middle continental shelf. A series of small-scale moraine ridges formed as the grounding line shifted south towards Roosevelt Island. The facies relationships on the middle continental shelf indicate that the calving front then shifted abruptly to its modern position. The ground-truth of the seafloor geomorphology presented here is important because it is a necessary framework for constraining retreat chronology for a central part of the WAIS that was not influenced by East Antarctic Ice Sheet overflow.

Salt Marsh and Fringing Oyster Reef Transgression in a Shallow Temperate Estuary: Implications for Restoration, Conservation and Blue Carbon

The importance of intertidal estuarine habitats, like salt marsh and oyster reef, has been well established, as has their ubiquitous loss along our coasts with resultant forfeiture of the ecosystem services they provide. Furthering our understanding of how these habitats are evolving in the face of anthropogenic and climate driven changes will help improve management strategies. Previous work has shown that the growth and productivity of both oyster reefs and salt marshes are strongly linked to elevation in the intertidal zone (duration of aerial exposure). We build on that research by examining the growth of marsh-fringing oyster reefs at yearly to decadal time scales and examine movement of the boundary between oyster reef and salt marsh at decadal to centennial time scales. We show that the growth of marsh-fringing reefs is strongly associated to the duration of aerial exposure, with little growth occurring below mean low water and above mean sea level. Marsh-shoreline movement, in the presence or absence of fringing oyster reefs, was reconstructed using transects of sediment cores. Carbonaceous marsh sediments sampled below the modern fringing oyster reefs indicate that marsh shorelines within Back Sound, North Carolina are predominantly in a state of transgression (landward retreat), and modern oyster-reef locations were previously occupied by salt marsh within the past two centuries. Cores fronting transgressive marsh shorelines absent fringing reefs sampled thinner and less extensive carbonaceous marsh sediment than at sites with fringing reefs. This indicates that fringing reefs are preserving carbonaceous marsh sediment from total erosion as they transgress and colonize the exposed marsh shoreline making marsh sediments more resistant to erosion. The amount of marsh sediment preservation underneath the reef scales with the reef’s relief, as reefs with the greatest relief were level with the marsh platform, preserving a maximum amount of carbonaceous sediments during transgression by buffering the marsh from erosional processes. Thus, fringing oyster reefs not only have the capacity to shelter shorelines but, if located at the ideal tidal elevation, they also keep up with accelerating sea-level rise and cap carbonaceous sediments, protecting them from erosion, as reefs develop along the marsh.

Climate-related response of dust flux to the central equatorial Pacific over the past 150 kyr

High resolution paleoclimate records from low latitudes are critical for understanding the role of the tropics in transmitting and generating feedbacks for high-latitude climate change on glacial–interglacial and millennial timescales. Here we present three new records of 230Thxs,0-normalized 232Th-derived dust fluxes from the central equatorial Pacific spanning the last 150 kyr at millennial-resolution. All three dust flux records share the “sawtooth” pattern characteristic of glacial–interglacial cycles in ice volume, confirming a coherent response to global climate forcing on long timescales. These records permit a detailed examination of millennial variability in tropical dust fluxes related to abrupt perturbations in oceanic and atmospheric circulation. Increases in dust flux in association with at least six of the longest Greenland stadials provide evidence that abrupt, high-latitude climate oscillations influenced the atmospheric aerosol load in the equatorial Pacific, with implications for both direct and indirect effects on the tropical energy balance. Our latitudinal transect of cores captures shifts in the position of the Intertropical Convergence Zone (ITCZ) in response to variations in the interhemispheric thermal gradient associated with cooling in Greenland and bipolar seesaw warming in Antarctica. These observations demonstrate that changes in the energy and hydrologic balance of the tropics were repeated features of the penultimate deglaciation, last glacial inception and last glacial cycle, and highlight the role of the tropical atmosphere as a dynamic and responsive component of Earth's climate system.

Glacial water mass structure and rapid δ18O and δ13C changes during the last glacial termination in the Southwest Pacific

Changes in ocean circulation are thought to have contributed to lowering glacial atmospheric CO2 levels by enhancing deep ocean sequestration of carbon that was returned to the atmosphere during glacial terminations. High-resolution benthic foraminiferal δ13C and δ18O records from a depth transect of cores in the Southwest Pacific Ocean presented here provide evidence that both wind- and thermohaline-driven circulation drove CO2 from the ocean during the last deglaciation. Shallow geochemical stratification in the glacial Southern Ocean was followed by a short pulse of rapid δ13C enrichment to intermediate water depths during Heinrich Stadial 1, indicative of better-ventilated intermediate waters co-occurring with documented wind-driven upwelling in the Southern Ocean. Intermediate depth δ13C enrichment paused at the start of the Antarctic Cold Reversal (∼14.7 ka), implying a brief shallow restratification, while deeper layers were progressively flushed of δ13C-depleted and δ18O-enriched waters, likely caused by the increasing influence of deep waters sourced from the North Atlantic. The coincidence of atmospheric CO2 increases with these geochemical shifts in both shallow and deep cores suggests that shifts in both atmospheric and oceanic circulation contributed to the deglacial rise of CO2.

Neodymium isotopic evidence for linked changes in Southeast Atlantic and Southwest Pacific circulation over the last 200 kyr

Knowledge of the geometry and strength of the deep overturning circulation is central to the understanding of past climate variability on glacial–interglacial (G–I) timescales. In this study, neodymium (Nd) isotopic ratios on planktonic foraminifera are used to reconstruct the water mass source and mixing history of intermediate and deep water in the Southeast (SE) Atlantic and Southwest (SW) Pacific over the past 200 kyr from five sediment cores. Nd isotopes from a depth transect of cores in the SE Atlantic displayed a stronger geochemical gradient around 3.5 km at the LGM, with higher εNd values of ∼−6.0 below that boundary than those of ∼−7.8 above. In contrast, a similar εNd value (∼−9.5) is observed at both the intermediate and abyssal depths in the Holocene. The glacial upper ocean is 1.8 εNd units less radiogenic than the lower ocean, reflecting an increase in the amount of North Atlantic Component Water (NACW) in the upper SE Atlantic. A coherent Nd isotope change was observed in the SE Atlantic and SW Pacific intermediate and deep water with major excursion to more radiogenic εNd values during Marine Isotope Stage (MIS) 2, 4 and 6. This suggests the flux of NACW to the Southern Ocean was reduced during cold marine stages, possibly accompanied by changes in the water mass geometry. The constant 2 εNd-unit offset between intermediate water in the SE Atlantic and SW Pacific suggests the persistent propagation of less radiogenic NACW to the upper Pacific Ocean over the last glacial cycle. Moreover, the larger glacial vertical εNd gradients between the intermediate and deep waters in both the SE Atlantic and SW Pacific indicate a decreased proportion of NACW entrainment into the deep oceans, consistent with studies proposing reduced mixing between the upper and lower meridional overturning cells.