Meltwater Pulse 1B Research Articles

Reply to comment by E. Bard et al. on “Younger Dryas sea level and meltwater pulse 1B recorded in Barbados reef crest coral Acropora palmata” by N. A. Abdul et al.

AbstractAbdul et al. (2016) presented a detailed record of sea level at Barbados (13.9–9 kyr B.P.) tightly constraining the timing and amplitude during the Younger Dryas and Meltwater Pulse 1B (MWP‐1B) based on U‐Th dated reef crest coral species Acropora palmata. The Younger Dryas slow stand and the large (14 m) rapid sea level jump are not resolved in the Tahiti record. Tahiti sea level estimates are remarkably close to the Barbados sea level curve between 13.9 and 11.6 kyr but fall below the Barbados sea level curve for a few thousand years following MWP‐1B. By 9 kyr the Tahiti sea level estimates again converge with the Barbados sea level curve. Abdul et al. (2016) concluded that Tahiti reefs at the core sites did not keep up with intervals of rapidly rising sea level during MWP‐1B. We counter Bard et al. (2016) by showing (1) that there is no evidence for a hypothetical fault in Oistins Bay affecting one of the Barbados coring locations, (2) that the authors confuse the rare occurrences of A. palmata at depths >5 m with the “thickets” of A. palmata fronds representing the reef‐crest facies, and (3) that uncertainties in depth habitat proxies largely account for differences in Barbados and Tahiti sea level differences curves with A. palmata providing the most faithful proxy. Given the range in Tahiti paleodepth uncertainties at the cored sites, the most parsimonious explanation remains that Tahiti coralgal ridges did not keep up with the sea level rise of MWP‐1B.

Comment on “Younger Dryas sea level and meltwater pulse 1B recorded in Barbados reefal crest coral Acropora palmata” by N. A. Abdul et al.

AbstractBased on new U‐Th ages of corals drilled offshore Barbados, Abdul et al. (2016) have confirmed the existence of the abrupt stratigraphic feature called meltwater pulse 1B (MWP‐1B), which they interpret as being due to a very large and global sea level step change dated at about 11.3 kyr before present (approximately 15 m and equivalent to twice the amount of water stored in the present Greenland ice sheet). This contrasts with the Tahiti record, in which MWP‐1B is essentially absent or very small, as Carlson and Clark (2012) and Lambeck et al. (2014) also conclude in their recent reviews of deglacial sea levels at the global scale. However, the evidence provided by Abdul et al. and their main conclusions are not convincing as they are affected by the following three main problems, which may explain the apparent discrepancies: Problem #1/Barbados is located in a subduction zone, which was also active throughout the Late Glacial period. Furthermore, the Barbados cores studied by Abdul et al. were drilled on both sides of the extension of a tectonic feature identified at the southern tip of Barbados (South Point) as underlined by several studies of the Barbados stratigraphy. Problem #2/Fossil samples of Acropora palmata may not be reliable sea level markers during rapid and large sea level rises. Indeed, the asexual reproduction strategy of this species may not be optimal to keep up when the water depth is increasing very rapidly. This may in part explain why the living depth of A. palmata at Barbados was significantly greater than 5 m during some periods of the last deglaciation, notably between 14.5 and 14 kyr B.P. and possibly between 14 and 11.5 kyr B.P. Problem #3/The slow glacio‐isostatic adjustment and the rapid responses due to gravitational changes of ice and water masses complicate the interpretation of individual relative sea level (RSL) records at specific locations. Therefore, the Barbados and Tahiti record cannot be compared directly in terms of absolute sea level values as done by Abdul et al. In addition, different glaciohydroisostatic adjustments at the two sites may also have contributed to the observed discrepancy between their deglacial RSL records.

Younger Dryas sea level and meltwater pulse 1B recorded in Barbados reef crest coral Acropora palmata

AbstractThe Younger Dryas climate event occurred during the middle of the last deglacial cycle and is marked by an abrupt shift in the North Atlantic polar front almost to its former glacial position, trending east to west. Using high‐precision and high‐accuracy U‐Th‐dated Barbados reef crest coral, Acropora palmata, we generate a detailed sea level record from 13.9 to 9000 years before present (kyr B.P.) and reconstruct the ice volume response to the Younger Dryas cooling. From the mid‐Allerød (13.9 kyr B.P.) to the end of the Younger Dryas (11.65 kyr B.P.), rates of sea level rise decreased smoothly from 20 mm yr−1 to 4 mm yr−1, culminating in a 400 year “slow stand” before accelerating into meltwater pulse 1B (MWP‐1B). The MWP‐1B event at Barbados is better constrained as beginning by 11.45 kyr B.P. and ending at 11.1 kyr B.P. during which time sea level rose 14 ± 2 m and rates of sea level rise reached 40 mm yr−1. We propose that MWP‐1B is the direct albeit lagged response of the Northern Hemisphere ice sheets to the rapid warming marking the end of the Younger Dryas coinciding with rapid warming in the circum‐North Atlantic region and the polar front shift from its zonal to meridional position 11.65 kyr B.P. As predicted by glaciological models, the ice sheet response to rapid North Atlantic warming was lagged by 400 years due to the thermal inertia of large ice sheets. The regional circum‐North Atlantic Younger Dryas climate event is elevated to a global response through sea level changes, starting with the global slowdown in sea level rise during the Younger Dryas and culminating with MWP‐1B. No meltwater pulses are evident at the initiation of the Younger Dryas climate event as is often speculated.

Submerged shoreline preservation and ravinement during rapid postglacial sea-level rise and subsequent “slowstand”

Submerged shorelines hold much potential for examining the interplay between the rate of sea-level rise and geomorphic setting, and informing the development of models of contemporary shoreline behavior. This paper describes the sedimentary architecture of a submerged barrier shoreline complex off Durban, South Africa. The complex consists of several barriers that have survived the postglacial transgression and associated erosive ravinement processes. The main shoreline sequence (–60 m) dates to 11,690 ± 90 calibrated (cal) yr B.P. and rests on a Pleistocene lagoonal deposit (35,395 ± 592 cal yr B.P.). The entire barrier shoreline complex is truncated by a strongly diachronous wave ravinement surface. The ravinement surface seaward of the main –60 m shoreline is steep, but the gradient declines across and landward of the subordinate landward shoreline complexes. The steep ravinement surface is attributed to fast sea-level rise (possibly associated with meltwater pulse 1B), while the gentle ravinement surface is associated with a subsequent slowing of the rate of sea-level rise (to 0.15 mm yr –1 ). Preservation of the main barrier and back-barrier deposits through overstepping is associated namely with rapid transgression and increased back-barrier accommodation. The smaller barriers landward of the main barrier were preserved through overstepping related to gentle antecedent gradients, despite more intense ravinement during a very slow rise in sea level (slowstand). This process was assisted by a sediment deficit. The resulting post-transgressive drape is also influenced by antecedent topography created by the barriers themselves; damming along the landward sides of overstepped barriers thickens the drape and creates a temporal disconnect between the migration of the shoreface and more landward elements of the littoral system. In examining the rates of shoreline migration associated with the overstepped barrier system, these are far greater than those calculated for the predicted rates of shoreline change on similarly steep coastal profiles. Future rates of shoreline migration appear to be insufficient to cause overstepping, and rollover or erosional responses are more likely.

Continental shelf morphology and stratigraphy offshore San Onofre, California: The interplay between rates of eustatic change and sediment supply

Abstract New high-resolution CHIRP seismic data acquired offshore San Onofre, southern California reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply. Throughout the majority of the study region, a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from ~ 72 to 53 m below present sea level. These erosional features appear to have formed between Melt Water Pulse 1A and Melt Water Pulse 1B, when the rate of sea-level rise was lower. There are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the Holocene transgressive surface in the seismic data. Unit I, the deepest unit, is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform. Unit I thins seaward by downlap and pinches out landward against the shoreline cutoff. Unit II is a mid-shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap. The youngest, Unit III, is interpreted to represent modern sediment deposition. Faults in the study area do not appear to offset the transgressive surface. The Newport Inglewood/Rose Canyon fault system is active in other regions to the south (e.g., La Jolla) where it offsets the transgressive surface and creates seafloor relief. Several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area. Nevertheless, our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression. The Cristianitos fault zone also causes a shoaling of the transgressive surface. This may be from resistant antecedent topography due to an early phase of compression on the fault. The Cristianitos fault zone was previously defined as a down-to-the-north normal fault, but the folding and faulting architecture imaged in the CHIRP data are more consistent with a strike-slip fault with a down-to-the-northwest dip-slip component. A third area of shoaling is observed off of San Mateo and San Onofre creeks. This shoaling has a constructional component and could be a relict delta or beach structure.

A post-glacial relative sea-level curve from Fiordland, New Zealand

The modern fjords of southwest New Zealand were previously stranded lakes isolated from the Tasman Sea by bedrock and moraine sills following the retreat of glaciers at the Last Glacial Maximum. The isolated lake basins were subsequently inundated with sea water when sea-level rise overtopped the sills. A record of the lacustrine-to-marine environmental transition is preserved in the fjord basin sediments and is identified in two New Zealand fjords with high-resolution seismic data and paleoenvironmental analysis of sediment cores. Seismic data are used to constrain the maximum sill depth and microfossil assemblages are used to track the lacustrine-to-marine transition. Chronology is based on fourteen radiocarbon ages.A relative sea-level curve for Fiordland, New Zealand is constructed based on sill depths and age constraints on the marine incursion. The sea-level curve allows insights into estimated uplift rates for Fiordland during the Holocene. From a lowstand of at least 107mbsl 14,750yr ago, these data reveal a stepwise transgression. Meltwater Pulse 1b is identified between 12,400 and 11,400yr ago, with a second acceleration in sea-level rise observed 9700yr ago. This record contributes a new sea-level curve for a mid-latitude (45°S) Southern Hemisphere location as well as new evidence for Meltwater Pulse 1b.

The lost Adventure Archipelago (Sicilian Channel, Mediterranean Sea): Morpho-bathymetry and Late Quaternary palaeogeographic evolution

The rise in sea level following the Last Glacial Maximum (LGM) radically altered pre-existing geography, pushing back the former shorelines. The effect of this eustatic change was particularly intense and dramatic in flat lands and shelves, such as the Sicilian Channel. High-resolution bathymetric maps and Chirp seismic profiles analyzed in this study show that the Sicilian Channel, now lying at depths rarely exceeding 150m, has emerged several times during its geological history. The last emergence was during the Early Holocene, when kilometre-sized islands punctuated the north-western sector of the Sicilian Channel, the Adventure Plateau, forming a broad archipelago. Many of these islands, now located in water ranging in depth from −10 to −40m, are composed of highly deformed Neogene sedimentary rocks (Talbot, Ante-Talbot, Nereo, and Pantelleria Vecchia banks), while others represent submarine Pleistocene volcanic edifices (Galatea, Anfitrite and Tetide banks). Sedimentary cover in all of these banks is virtually absent. High-resolution seismic profiles allowed the identification of post-LGM morphological markers associated with the marine transgression, some of which are characterized by melt water-pulses, as demonstrated by the presence of specific and distinct erosional features. Combining swath bathymetric data with the seismic profiles, we have generated two palaeogeographic maps of the former Adventure Archipelago at two specific time frames: (1) at the end of the Younger Dryas stadial (11,500yr B.P., corresponding to a former sea level of −60m), and (2) at the end of the melt-water pulse 1B (11,200yr B.P., corresponding to a former sea level of −42m). Maps clearly show that in just 300years the geography of the archipelago has changed dramatically, so much so that some islands have disappeared and some have decreased by more than 80% of their pre-Younger Dryas size.

Fine sediment mineralogy as a tracer of latest Quaternary sediment delivery to a dynamic continental margin: Pandora Trough, Gulf of Papua, Papua New Guinea

Sediment cores and geophysical data from the shelf and slope in the Pandora Trough and Gulf of Papua (GoP; Papua New Guinea, northern Coral Sea) have been studied to evaluate timing, pathways, and sources of sediment delivery to a muddy shelf-slope depositional system that developed during the late Quaternary marine transgression. Cores were analyzed for 14C geochronology, clay and bulk mineralogy, grain size, sedimentary fabric, and physical properties. Observations reveal a shelf-edge depocenter and adjacent middle-to-upper slope sediment wedge that each developed at different times from different sedimentary sources, despite close proximity along a down-slope gradient. Clay mineralogy of modern shelf sediments reveals strong longitudinal gradients that appear related to fluvial source composition. Similar gradients in source composition reveal heterogeneous and shifting sediment sources to the shelf and slope through time. From the Bølling–Allerød interstadial (14.5–12.5 kyr BP), and the Younger Dryas (12.5–11.5 kyr BP), fluvial sediments accumulated in a muddy inner shelf, forming a shelf-edge depocenter (core MV-41) with high kaolinite/chlorite values that are indicative of a local volcanic source, consistent with the geology of the nearby Lakekamu and Vailala river catchments. Shelf-edge sediment accumulation decreased from the time of Meltwater Pulse 1B (MWP-1B) to the present due to shelf flooding and inshore trapping of sediment. During MWP-1B, and possibly earlier, sediment transport west of MV-41 was steered obliquely offshore to the northern Pandora Trough continental margin upper/middle slope (MV-46 and MV-49). This transport has persisted nearly to the present, and has allowed for the development of a Holocene upper/middle slope sediment wedge (cores MV-46 and MV-49) that differs strongly in clay composition (and so source) from the adjacent up-slope shelf-edge depocenter (MV-41). Slope deposits have intermediate illite/smectite values similar to the central GoP, which differ from the very low illite/smectite values at MV-41. Such heterogeneous dispersal patterns oblique to seabed gradient are likely due to complex interactions of shelf processes with dynamic shelf morphology along the flooding GoP shelf, rather than a response solely due to rising sea-level and down-slope transport.

Rapid flooding of the southern Vietnam shelf during the early to mid‐Holocene

ABSTRACTA new sea‐level record derived from coastal deposits of the South Vietnam shelf covers the deglacial sea‐level history between 13.0 and 9.0 ka BP. This record reveals a relatively constant rate of sea‐level rise and matches well with model results based on the RSES‐ANU global ice‐sheet model. However, the sea‐level observations reveal a significant discrepancy with model results based on the ICE‐5G global ice‐sheet model, which further questions the existence of Meltwater Pulse 1B The incised‐valley deposits of the South Vietnam shelf also complete the deglacial coastal evolution of southern Vietnam. The regional sea‐level records of the South Vietnam shelf, the Mekong Delta and the Cambodian lowlands reveal an offset of about 18 m between 9.0 and 8.2 ka BP. The shoreline retreat of more than 200 km related to this offset was mainly caused by the differential hydro‐isostatic adjustment between the South Vietnam shelf and the Cambodian lowlands. However, the GIA‐model results cannot fully explain this fast relative sea‐level rise due to the topographic changes and, possibly, alternative melt‐water sources not accounted for in the current models.

Geomorphic and stratigraphic signals of postglacial meltwater pulses on continental shelves

Selective development and preservation of major shoreline complexes on the continental shelf provide geomorphic evidence in support of alternating periods of Holocene sea-level stillstand and rapid rise during meltwater pulses. On a tectonically stable far-fi eld setting (the southeast African shelf), regionally developed, well-preserved shoreline complexes occur at ~‐100 m and ~‐60 m within an overall shelf sequence dominated by transgressive ravinement (non-preservation of shorelines). The development of geomorphically mature submerged shorelines with equilibrium forms is attributed to extended periods of sea-level stability, and their preservation is the result of early cementation followed by very rapid sea-level rise (several centimeters per year) that caused overstepping. Meltwater Pulses 1A and 1B are recorded in the shelf stratigraphy and geomorphology. Setting aside local infl uences of topography and sediment supply, we hypothesize that shelf stratigraphies should preferentially preserve shorelines at water depths associated with the stillstands or slowstands immediately preceding meltwater pulses. A reappraisal of shelf stratigraphy and geomorphology, in particular the preservation potential of former shorelines, is required in the light of contemporary understanding of postglacial sea-level history.

Submerged barrier shoreline sequences on a high energy, steep and narrow shelf

Abstract Late Pleistocene–Holocene shoreline sequences and associated shelf stratigraphy are described from a high gradient, high wave-energy shelf. Although shoreline preservation by overstepping is considered unlikely in such an environment as a result of intense ravinement during shoreline translation, the development and preservation of two distinct shorelines at − 100 m and − 60 m are evident and attributed to century to millennial scale periods of stillstand. During these periods shoreline equilibrium forms developed, characterised by early diagenesis of beachrock and aeolianite. Between these features the shelf is dominated by erosional surfaces reflecting ravinement during periods of slowly rising sea-level. Where shorelines are not preserved, areas of scarping in the ravinement surface, at similar depths to the adjoining shorelines, represent rocky headlands separating the sandy coastal compartments where the depositional shorelines formed. Shoreline preservation by overstepping was facilitated by rapid relative sea-level rise associated with meltwater pulses 1A and 1B, although the features preserved are likely to represent only the cemented cores of the coastal dunes and linear beachrock outcrops formed. The steep antecedent shelf gradient fostered the erosion of significant portions of the shorelines by ravinement, with loose sediment re-organised into a thick post-ravinement drape. The removal of shorelines during ravinement was countered by the rapid rate of sea-level rise and early sub-tropical diagenesis. It is clear that due to the combined effects of subtropical diagenesis, long periods of stillstand and drowning by meltwater pulses, the governing controls on shoreline preservation can be over-ridden by combinations of local (climatic) and global (eustatic) factors.

テクトニックな沈降域における沿岸河口低地の地層形成

There have been fewer studies on strata formation in tectonically subsiding areas than in tectonically stable and uplifting areas. In this study, (1) 1000 year scale sediment stacking patterns, (2) seismic subsidence and meltwater pulse (MWP) 1B, and (3) event stratigraphy derived from seismic subsidence are revealed on the basis of sedimentary facies and 162 radiocarbon dating of three sediment cores obtained from the latest Pleistocene to Holocene Shinano River incised-valley fill (Alluvium) in the Echigo Plain. Aggradation is dominant in sediment stacking patterns of the incised-valley fill due to tectonic subsidence from the Nagaoka Plain Western Margin Fault Zone. Two continuous layers with abundant marine diatoms and rapid sediment accumulation rates are identified in transgressive salt marsh sediments. These layers may be due to seismic subsidence, but the latter layer (11.2-11.6 cal kyr BP) coincides with MWP 1B. If MWP 1B exists, the thickness of the latter layer indicates that the magnitude of the relative sea-level jump is less than 4 m. The event stratigraphy derived from seismic subsidence in salt marsh sediments is characterized by grading, dominant marine diatoms, and intense sediment accumulation rates. The presence of this event stratigraphy in the Echigo Plain suggests that seismic events strongly affect strata formation in tectonically subsiding areas in Japan.

Deglacial history of the West Antarctic Ice Sheet in the western Amundsen Sea Embayment

The Amundsen Sea Embayment (ASE) drains approximately 35% of the West Antarctic Ice Sheet (WAIS) and is one of the most rapidly changing parts of the cryosphere. In order to predict future ice sheet behaviour, modellers require long-term records of ice-sheet melting to constrain and build confidence in their simulations. Here, we present detailed marine geological and radiocarbon data along three palaeo-ice stream tributary troughs in the western ASE to establish vital information on the timing of deglaciation of the WAIS since the Last Glacial Maximum (LGM). We have undertaken multi-proxy analyses of the cores (core description, shear strength, x-radiographs, magnetic susceptibility, wet bulk density, total organic carbon/nitrogen, carbonate content and clay mineral analyses) in order to: (1) characterise the sedimentological facies and depositional environments; and (2) identify the horizon(s) in each core that would yield the most reliable age for deglaciation. In accordance with previous studies we identify three key facies, which offer the most reliable stratigraphies for dating deglaciation by recording the transition from a grounded ice sheet to open marine environments. These facies are: i) subglacial, ii) proximal grounding line, and iii) seasonal open marine. In addition, we incorporate ages from other facies (e.g., glaciomarine diamictons deposited at some distance from the grounding line, such as glaciogenic debris flows and iceberg-rafted diamictons and turbates) into our deglacial model. In total, we have dated 78 samples (mainly the acid insoluble organic (AIO) fraction, but also calcareous foraminifers), which include 63 downcore and 15 surface samples. Through careful sample selection prior to dating, we have established a robust deglacial chronology for this sector of the WAIS. Our data show that deglaciation of the western ASE was probably underway as early as 22,351 calibrated years before present (cal yr BP), reaching the mid-shelf by 13,837 cal yr BP and the inner shelf to within c.10–12 km of the present ice shelf front between 12,618 and 10,072 cal yr BP. The deglacial steps in the western ASE broadly coincide with the rapid rises in sea-level associated with global meltwater pulses 1a and 1b, although given the potential dating uncertainty, additional, more precise ages are required before these findings can be fully substantiated. Finally, we show that the rate of ice-sheet retreat increased across the deep (up to1600 m) basins of the inner shelf, highlighting the importance of reverse slope and pinning points in accelerated phases of deglaciation.

Relative sea-level change in and around the Younger Dryas inferred from late Quaternary incised-valley fills along the Japan Sea

The lastdeglacial sea-level change is characterized by episodes of rapid sea-level rise such as meltwater pulses (MWPs) and periods of slower sea-level rise such as the Younger Dryas (YD) period. However, the existence of MWP 1B, which occurred right after the YD at around 11,000 cal BP, has been debated for the last decade. The Echigo Plain, along the Japan Sea, is governed by an active fault along its western margin, and the latest Pleistocene to Holocene sedimentary fills in the buried incised valley of the Shinano River comprise intertidal salt marsh sediments aggraded during the last transgression. The present tidal range along the Japan Sea is less than 0.3 m. Therefore, eustatic sea-level trends and events such as MWP 1B are detectable from the incised-valley fills if the history of subsidence along the fault is constrained. In this study, the relative sea-level change during 13,000–9000 cal BP is reconstructed on the basis of radiocarbon dates and the depth of salt marsh sediments identified in two sediment cores obtained from the Echigo Plain. If it is assumed that the interseismic subsidence rate with sediment compaction effect of 2.6–3.7 mm/yr, calculated from the age and depth of sediments in the cores, was constant during the valley-fill deposition, five segments of rapid sea-level rise and five segments of slow sea-level rise during 13,000–9000 cal BP are detectable. Comparing relative sea-level curves (corrected for tectonic movements and sediment compaction effects) from the Echigo Plain, Barbados, and Tahiti, a rapid sea-level rise just before the YD and slow sea-level rise during the YD are found as common features of the three sites. However, three episodes of rapid sea-level rise at 11,000–10,900, 10,600–10,400 and 9900–9700 cal BP are found only for the Echigo Plain; these provide evidence of coseismic subsidence along the fault. The rapid sea-level rise during 11,400–11,300 cal BP on the Echigo Plain is consistent with MWP 1B in Barbados, but it can just as readily be attributed to coseismic subsidence, and if MWP 1B is real, its magnitude is only less than 4 m in the Japan Sea.

The last glacial–interglacial transition (LGIT) in the western mid-latitudes of the North Atlantic: Abrupt sea surface temperature change and sea level implications

High resolution reconstructions of sea surface temperature (U k′ 37-SST), coccolithophore associations and continental input (total organic carbon, higher plant n-alkanes, n-alkan-1-ols) in core D13882 from the shallow Tagus mud patch are compared to SST records from deep-sea core MD03-2699 and other western Iberian Margin cores. Results reveal millennial-scale climate variability over the last deglaciation, in particular during the LGIT. In the Iberian margin, Heinrich event 1 (H1) and the Younger Dryas (YD) represent two extreme episodes of cold sea surface condition separated by a marine warm phase that coincides with the Bølling–Allerød interval (B–A) on the neighboring continent. Following the YD event, an abrupt sea surface warming marks the beginning of the Holocene in this region. SSTs recorded in core D13882 changed, however, faster than those at deep-sea site MD03-2699 and at the other available palaeoclimate sequences from the region. While the SST values from most deep-sea cores reflect the latitudinal gradient detected in the Iberian Peninsula atmospheric temperature proxies during H1 and the B-A, the Tagus mud patch (core D13882) experienced colder SSTs during both events. This is most certainly related to a supplementary input of cold freshwater from the continent to the Tagus mud patch, a hypothesis supported by the high contents of terrigenous biomarkers and total organic carbon as well as by the dominance of tetra-unsaturated alkenone (C 37:4) observed at this site. The comparison of all western Iberia SST records suggests that the SST increase that characterizes the B-A event in this region started 1000 yr before meltwater pulse 1A (mwp-1A) and reached its maximum values during or slightly after this episode of substantial sea-level rise. In contrast, during the YD/Holocene transition, the sharp SST rise in the Tagus mud patch is synchronous with meltwater pulse 1B. The decrease of continental input to the mud patch confirms a sea level rise in the region. Thus, the synchronism between the maximum warming in the mid-latitudes off the western Iberian margin, the adjacent landmasses and Greenland indicates that mwp-1B and the associated sea-level rise probably initiated in the Northern Hemisphere rather than in the South.

Geomorphology and benthic cover of mesophotic coral ecosystems of the upper insular slope of southwest Puerto Rico

The upper insular slope of southwest Puerto Rico is defined as extending from the shelf break at ~20 m water depth down to a depth of ~160 m where there is a pronounced change in geomorphic character and the basal slope begins. The upper slope is divided into two geomorphic zones separated by a pronounced break in slope gradient at ~90 m water depth. Descending from the shelf break, these are Zone I (20–90 m) and Zone II (90–160 m). As orientation of the shelf margin changes, geomorphology of Zone I shows systematic variations consistent with changes in exposure to prevailing waves. Within Zone I, exposed southeast-facing slopes have a gentler gradient and lower relief than more sheltered southwest-facing slopes, which are steep and irregular. Mesophotic coral ecosystems (MCEs) are largely restricted to Zone I and concentrated on topographic highs removed from the influence of active downslope sediment transport. Accordingly, MCEs are more abundant, extensive and diverse on southwest-facing slopes where irregular topography funnels downslope sediment transport into steep narrow grooves. MCEs are more sporadic and widely spaced on southeast-facing slopes where topographic highs are more widely spaced and downslope sediment transport is spread over open, low-relief slopes inhibiting coral recruitment and growth. Relict features formed during preexisting sea levels lower than present include deep buttresses at ~45–65 m water depth and a prominent terrace at ~80 m. Based on correlations with existing reef accretion and sea-level records, it is proposed that the 80-m terrace formed during the last deglaciation ~14–15 ka and subsequently drowned during a period of rapid sea-level rise associated with meltwater pulse 1A at ~14 ka and deep buttresses at ~45–65 m formed between ~11.5 and 13.5 ka and then drowned during a period of rapid sea-level rise associated with meltwater pulse 1B at ~11.3 ka.

Deglacial Meltwater Pulse 1B and Younger Dryas Sea Levels Revisited with Boreholes at Tahiti

Reconstructing sea-level changes during the last deglaciation provides a way of understanding the ice dynamics that can perturb large continental ice sheets. The resolution of the few sea-level records covering the critical time interval between 14,000 and 9,000 calendar years before the present is still insufficient to draw conclusions about sea-level changes associated with the Younger Dryas cold event and the meltwater pulse 1B (MWP-1B). We used the uranium-thorium method to date shallow-living corals from three new cores drilled onshore in the Tahiti barrier reef. No significant discontinuity can be detected in the sea-level rise during the MWP-1B period. The new Tahiti sea-level record shows that the sea-level rise slowed down during the Younger Dryas before accelerating again during the Holocene.

The sea-level record of the last deglacial in the Shinano River incised-valley fill, Echigo Plain, central Japan

The Echigo Plain, on the Japan Sea coast, is in a sedimentary basin governed by subsidence controlled by the Echigo Plain Western Margin Fault Zone. The glacial incised valley in the Echigo Plain filled with the sediment discharge of the Shinano River as the sea level rose during the last deglacial. The incised-valley fill allows us to reconstruct the detailed sea-level record, because the plain is still interseismically subsiding at a rate of about 3 mm/yr and the present tidal range along the Japan Sea coast is less than 0.3 m. In this study, we reconstructed a centennial-scale sea-level record for the deglacial period (12,000–9000 cal BP) on the basis of the sedimentary facies and 54 radiocarbon dates from a sediment core obtained from the Echigo Plain near the Western Margin Fault Zone. In this core, the interval from 95 to 55 m below the present sea level consists of floodplain and salt marsh sediments deposited aggradationally during 11,600–9000 cal BP as a result of transgression. Taking into account the Holocene subsidence rate of 2.7 mm/yr at the core site, the relative sea-level rise during the period of core deposition can be divided into the periods of rapid and slow rates. Sea level rose rapidly (> 23 mm/yr) during 11,600–11,100 cal BP, followed by a slow rate of sea-level rise (2 mm/yr) during 11,200–10,300 cal BP. Another period of rapid sea-level rise (11 mm/yr) during 10,500–9400 cal BP was followed by a slow rate of rise (2 mm/yr) during 9600–9000 cal BP. This record of relative sea-level rise indicates that the rapid rate of sea-level rise during 11,600–11,100 cal BP is consistent with meltwater pulse 1B.

Bundled turbidite deposition in the central Pandora Trough (Gulf of Papua) since Last Glacial Maximum: Linking sediment nature and accumulation to sea level fluctuations at millennial timescale

Since Last Glacial Maximum (23–19 ka), Earth climate warming and deglaciation occurred in two major steps (Bølling‐Allerød and Preboreal), interrupted by a short cooling interval referred to as the Younger Dryas (12.5–11.5 ka B.P.). In this study, three cores (MV‐33, MV‐66, and MD‐40) collected in the central part of Pandora Trough (Gulf of Papua) have been analyzed, and they reveal a detailed sedimentary pattern at millennial timescale. Siliciclastic turbidites disappeared during the Bølling‐Allerød and Preboreal intervals to systematically reoccur during the Younger Dryas interval. Subsequent to the final disappearance of the siliciclastic turbidites a calciturbidite occurred during meltwater pulse 1B. The Holocene interval was characterized by a lack of siliciclastic turbidites, relatively high carbonate content, and fine bank‐derived aragonitic sediment. The observed millennial timescale sedimentary variability can be explained by sea level fluctuations. During the Last Glacial Maximum, siliciclastic turbidites were numerous when the lowstand coastal system was located along the modern shelf edge. Although they did not occur during the intervals of maximum flooding of the shelf (during meltwater pulses 1A and 1B), siliciclastic turbidites reappear briefly during the Younger Dryas, an interval when sea level rise slowed, stopped, or perhaps even fell. The timing of the calciturbidite coincides with the first reflooding of Eastern Fields Reef, an atoll that remained exposed for most of the glacial stages.

Last glacial–Holocene water structure in the southwestern Okhotsk Sea inferred from radiolarian assemblages

In order to reconstruct paleoceanographic conditions in the southwestern (SW) Okhotsk Sea, radiolarian assemblages were analyzed from 10 surface sediments and 5 sediment cores obtained from various water depths, ranging between 461 and 1348 m. We also measured geochemical components such as biogenic opal, calcium carbonate, total organic carbon, total sulfur and total nitrogen for one sediment core. These data imply that the complicated water structure in the SW Okhotsk Sea changed with time from the last glacial to Holocene. The glacial surface water was characterized by low primary production in the summer and expanded sea-ice coverage in the winter. During this time, the ventilation might have reached deeper than during the Holocene. During the major deglacial period including the melt-water pulses 1A and 1B, the deep-sea oxygen content had declined with less ventilation and/or more oxygen consumption with organic matter supply into the deeper depths. The intermediate layer has been well ventilated and has supplied large amount of organic matter during the last 20 kyrs, especially in the early-mid Holocene, however such condition ceased during 3 to 2 ka. The warm Pacific deep water has been present since 9 ka. The influence of warmer surface water such as the Soya Warm Current (SWC) increased in the coastal area during 7 to 4 ka and since 2 ka till present. Neritic-derived productivity might be extended at 2.2–1.8 ka and since 1.2 ka till present.