Meltwater Pulse 1B Research Articles

The hemispheric origins of meltwater pulse 1B

SUMMARY Antarctica has been proposed as a significant source of the meltwater that entered the oceans during meltwater pulse 1B (MWP1B) approximately 11 500 yr ago. Support for this scenario has been provided by evidence that the deep fjords of coastal Antarctica, which were heavily glaciated at the maximum of glaciation, were deglaciated at this time. Further support for this scenario was provided by the observation that the inter-hemispheric sea-level teleconnection associated with significant Southern Hemisphere deglaciation at this time provided an explanation of the highly non-monotonic relative sea-level histories recorded at sites on the coast of Scotland, a region which had also been heavily glaciated at the last glacial maximum. Furthermore, it has been argued that a significant contribution to MWP1B must have also been delivered to the oceans by the abrupt Northern Hemisphere warming that occurred at the end of the Younger Dryas (YD) cold reversal, which also occurred approximately 11 500 yr ago. Our focus in this paper is to distinguish between these two possible primary sources of MWP1B. The investigation of how local alterations to ice thicknesses are able to explain evidence which has previously been used to argue for an Antarctic dominant MWP1B will lead us to the conclusion that the Laurentide may be primary source of MWP1B.

Sedimentary evolution of the western North Yellow Sea mud patch: A Holocene perspective

The western North Yellow Sea mud patch has a unique location, abundant sediment supply and a complex sedimentary environment. Reconstructing its sedimentary evolution is crucial for a comprehensive understanding of source to sink process in the North Yellow Sea. Using core W03 collected from the mud patch, this study unravels its Holocene history through analysis of sedimentology, mineralogy and micropalaeontology. The results indicate that core W03 can be divided into three depositional units (DU). DU3 (538-461 cm) is composed of earthy yellow clayey silt and characterized by a hard layer. It was formed in the near-coast environment between 10.3 and 9.8 cal kyr BP, a hiatus period between Melt Water Pulse 1B and 1C events. DU2 (460-341 cm) was formed during the period of 9.8–6.5 cal kyr BP. As sea level rose, the westward retreat of the Yellow River mouth caused the transportation of Yellow River-derived sediment over longer distances, leading to an increase in flaky minerals. The rising sea level also induced a shift towards a reducing environment in the western North Yellow Sea. DU1 (340-0 cm) was formed after 6.5 cal kyr BP. During this period, the study area remained in a shelf shallow sea environment characterized by relatively strong chemical hydrolysis and reducing conditions. The intensification of the Yellow Sea Warm Current and an increase in Yellow River sediment transported to the sea led to an obvious increase in sedimentation rate after 2.3 cal kyr BP.

Episodic postglacial deltaic pulses in the Gulf of Cadiz: Implications for the development of a transgressive shelf and driving environmental conditions

Abstract The postglacial sea-level rise after the Last Glacial Maximum provided ideal conditions to study the transgressive sedimentary response to sudden shelf flooding driven by different rates of sea-level rise. In this study, a high-resolution seismic stratigraphic interpretation and sedimentological analysis were conducted on data from the northern Gulf of Cadiz continental shelf (SW Iberian Peninsula), in order to: 1) understand the succession of sedimentary processes during each shelf flooding episode and 2) explore the significance of variable rates of sea-level rise, sediment fluxes, and climatic conditions on the development of postglacial deposits. Four backstepping seismic postglacial transgressive units (PTUs; 4 to 1 from oldest to youngest) that are linked to the retreating mouth of the Guadiana River were interpreted. Together, these seismic units display a wedge-shape geometry, are located over the inner to middle shelf, and overlie a regional unconformity formed during the Last Glacial Maximum. Each PTU can be divided into several sub-units with distinctive seismic facies that have a similar stratigraphic organization. Each PTU contains lower sub-units that are composed of low-angle tangential-oblique clinoforms. The clinoforms are locally topped by a channelized sub-unit. The distal and/or lateral parts of the clinoforms are occasionally buried by sheet-like semitransparent subunits. The uppermost sub-units are present over the proximal and central parts of each seismic unit and are also sheet-like. PTUs can also be subdivided and described sedimentologically. Fine-grained sands with intercalated silty layers dominate the lower part of each PTU (lower clinoform sub-units). The upper part of each PTU (upper sheet-like sub-units) is characterized by reworked facies, composed of highly fragmented bioclasts within a mixture of silt and coarse to medium sand. Finally, mud deposits occur as a sediment drape over the PTUs. The internal structure of each PTU reveals several phases of development under a general process of transgressive submergence in which both coastal and marine deposits were formed and eventually preserved. The initial phase involved the development of coarse-grained deltas in shallow water, which were locally eroded by a network of distributary channels. In a transitional phase, the infilling of distributary channels and the offshore export of fine-grained sediments is related to a change in sediment sources, possibly triggered by enhanced hydrodynamic processes. The final phase involved the reworking of fluvio-deltaic sediments by shoreface processes to generate a sediment sheet. Age correlation with a suite of postglacial sea-level curves indicates that the formation of the postglacial transgressive deposits is bracketed between 14 ka and 9 ka. The studied deposits are related to a period of reduced sea-level rise, culminating in the Younger Dryas event (two oldest PTUs), and to phases of enhanced sea-level rise, such as Meltwater Pulse (MWP) 1B (two youngest PTUs). In spite of high rates of sea-level rise over MWP-1B, each PTU exhibits progradation and preservation of much of the delta. The preservation of progradational deltaic units is likely caused by increased sediment supply during progradational pulses. We suggest that those pulses of enhanced sediment fluxes during MWP-1B were strongly driven by the overall climatic conditions in the southwest of the Iberian Peninsula, probably resulting from enhanced rainfall runoff during humid periods and scarce land vegetation cover.

Deglacial release of petrogenic and permafrost carbon from the Canadian Arctic impacting the carbon cycle

The changes in atmospheric pCO2 provide evidence for the release of large amounts of ancient carbon during the last deglaciation. However, the sources and mechanisms that contributed to this process remain unresolved. Here, we present evidence for substantial ancient terrestrial carbon remobilization in the Canadian Arctic following the Laurentide Ice Sheet retreat. Glacial-retreat-induced physical erosion of bedrock has mobilized petrogenic carbon, as revealed by sedimentary records of radiocarbon dates and thermal maturity of organic carbon from the Canadian Beaufort Sea. Additionally, coastal erosion during the meltwater pulses 1a and 1b has remobilized pre-aged carbon from permafrost. Assuming extensive petrogenic organic carbon oxidation during the glacial retreat, a model-based assessment suggests that the combined processes have contributed 12 ppm to the deglacial CO2 rise. Our findings suggest potentially positive climate feedback of ice-sheet retreat by accelerating terrestrial organic carbon remobilization and subsequent oxidation during the glacial-interglacial transition.

Late Pleistocene to Holocene glacio-eustatic history as recorded in the Pescara paleovalley system (Central Italy, Adriatic basin)

A buried paleovalley system, about 50 m deep and 2 km wide, is documented from the Pescara coastal plain. Based on stratigraphic, sedimentological, paleontological, chronological and geotechnical data, the paleovalley profile and 3D facies architecture of the paleovalley fill (PVF) were reconstructed.The lowermost PVF is a laterally extensive fluvial gravel body, up to 13 m-thick, that represents the lowstand systems tract (LST; pre-11.3 ka cal BP). Above lowstand deposits, the transgressive systems tract (TST), 21 m-thick, shows a deepening-upward trend, from freshwater/inner-estuarine to brackish/outer-estuarine facies associations (11.3–8.0 ka cal BP). The upper part of the succession (highstand systems tract – HST) shows a shallowing-upward tendency from paludal to fluvio-deltaic deposits.Seven millennial-scale parasequences (Ps) were identified within the Pescara Holocene (TST + HST) succession. Transgressive Ps1-3 exhibit a distinctive retrogradational stacking pattern. Highstand Ps4-7 are aggradationally-to-progradationally stacked. During the aggradational phase (P4), the estuary was gradually filled and swamp environments spread onto the valley interfluve. Because of subsequent progradation (Ps5-7), delta plain conditions established. In the research core, TST parasequences show higher accumulation rates (up to 9.4 mm/y) than HST ones (1.3–1.8 mm/y). Thus, the study area evolved from a region of sediment storage (11.3–8.0 ka cal BP) into a sector of prevalent sediment bypass (last 8.0 ky).Major early Holocene flooding events were possibly triggered by Melt-Water Pulses (MWPs) 1B, 1C and 1D. The eustatic rise linked to MWP-1B reasonably caused the P1 flooding event (11.3 ka cal BP). Post-MWPs 1C and 1D sea-level rises likely provoked the complete drowning of the paleovalley system and the subsequent maximum landward migration of the shoreline (about 8.0 ka cal BP).This study provides new evidences, in terms of sedimentary response, of the poorly-documented MWPs 1C and 1D, and the first documentation of MWP-1B eustatic effects in an onshore sector of the Central Adriatic.

Sedimentological and morphological evidences of Meltwater Pulse 1B in the Southwestern Atlantic Margin

During the post-LGM deglaciation, global temperatures and rates of sea-level rise were influenced by centennial events driven by climate and ice volume variations, leading to cooling/warming climate stages, sea-level slowstands and meltwater pulses (MWP). Sea level fluctuations have shaped the shelf morphology by drowning coastal features and imposing shifts in coastal/shelf sedimentary environments worldwide. The influence of meltwater pulses in the South Atlantic is still not well established. Thus, here, we investigate morphological and sedimentological records of MWP-1B along two distinct areas of the Eastern Brazilian shelf (Espirito Santo and Abrolhos shelf), far-field sites in the western South Atlantic. Sedimentary core analysis, C14 dating, and multibeam data are combined to provide stratigraphic and morphological evidences of sea-level rise acceleration at the end of Younger Dryas, in the South Atlantic. Results of core analysis indicate a coastal environment around 12 kyrs BP and a major shift in sedimentary facies after 11.5 kyrs BP. In both cores, Unit 1 comprises terrigenous (low carbonate content) and mixed facies, and are interpreted as terrigenous/coastal/estuarine facies. Unit 2 is characterized by carbonate/mixed facies, interpreted as open marine deposits. Terrigenous and mixed estuarine/coastal facies (Unit 1) are replaced by open marine facies (unit 2) at 57.3 m in the Abrolhos Depression core (ABDR) and at 47.6 m in the Espirito Santo paleovalley shelf (PVS core). Morphological evidence shows a paleoshoreline at 60 m water depth associated with five drowned and partially filled incised shelf valleys. A depth x age analysis indicates that the sedimentary environment shift and the depth of paleoshorelines are associated with sea-level slowstand during Younger Dryas, followed by a sea-level rise acceleration, representing MWP-1B. These evidences in the western South Atlantic are combined with other morphological and stratigraphic results presented for South Hemisphere far field sites, indicating that the 60 m isobath is an average water depth marking sea-level stabilization during Younger Dryas, and that MWP-1B is potentially a global event.

Sedimentary signatures of the abrupt deglacial rise in sea level from the East China Sea inner shelf

Understanding the link between deglacial sea-level jumps and abrupt climate change may provide crucial insights into future ice-climate feedbacks. However, much ambiguity remains surrounding many of the last deglacial meltwater pulses. Here, we present a complete sedimentary succession from the inner shelf of the East China Sea, which has clearly documented multiple abrupt rises in the deglacial sea level. The results suggest that the postglacial sedimentary sequence, resting on a late Pleistocene basement of stiff clay, formed during a three-stage process marked by characteristic lithology and foraminiferal fauna. The abrupt shifts in the sedimentary facies indicate that seawater firstly intruded into the inner shelf just before the onset of the slowdown in sea-level rise caused by the Younger Dryas cooling event and changed the inner shelf into a tidal flat environment, as demonstrated by the sudden appearance of both planktonic foraminifera and typical brackish-water species of benthic foraminifera. Meltwater pulse 1B (MWP-1B) is depicted by a sharp upward transition to a nearshore subtidal environment at 11.62 kyr. This episode of sea-level rise induced the initial flooding of the inner shelf, which is well documented by the sudden reduction in brackish-water foraminifera species and evident increase in inner-shelf benthic species. The prominent sea-level rise at approximately 7.54 kyr was responsible for the shift to stable inner shelf conditions and resulted in the maximum flooding of the inner shelf area, initiating subsequent formation of the alongshore mud wedge. Additionally, obvious peaks in both the benthic foraminifer abundance and the marine algae concentration at approximately 8.13 kyr confirm the previously identified sea-level jump at approximately 8.2 kyr, which coincides with the weakening of the Asian monsoon demonstrated by the peak grain size in our record, further indicating its apparent connection to the 8.2 kyr climatic reversal.

Response of a wave-dominated coastline and delta to antecedent conditioning and fluctuating rates of postglacial sea-level rise

In the context of rising sea levels, wave-dominated coasts and deltas are among the most threatened of coastal landforms. The study of submerged deltas, in tandem with other drowned wave-dominated shoreline sequences, can shed light on the long-term coastal behaviour of these threatened shorelines when subject to high-end scenarios of relative sea-level rise. Using ultra high-resolution seismic reflection data and gravity core samples, this paper investigates the post-glacial stratigraphic development and architecture of the shelf off southern Mozambique. We document several well-preserved beachrock/aeolianite palaeo-shoreline complexes from water depths of −100, −75 and −60 m, interpreted as representing the positions of former sea-level slowstands or stillstands. These overlie the regional subaerial unconformity associated with the last glacial maximum and are linked to two phases of deltaic deposition. Pro-deltaic deposits on the outer shelf were linked with the period of sea-level stability attributed to the −100 m shoreline, while proximal delta front deposits on the inner- to mid-shelf are linked to a slowstand evident in the local sea level record at −40 m. Well-developed lower delta plain facies overlie both shoreline and delta front facies and show evidence of strong channelisation during normal regression of the delta. These are overlain across the shelf by isolated and stranded pieces of the now relict shoreface, trapped to seaward by the shoreline complexes. The more proximal parts of the shelf are in turn overlain by fluvial-derived muddy facies of the current prodeltas of the area.The preservation of shoreline and delta deposits was enhanced by intervening rapid increments in sea-level rise associated with meltwater pulses 1A (−100 m) and 1B,(−60 m) though the preservation of the −75 shoreline feature, in spite of modest rates of Older Dryas base-level change, points toward other factors such as rapid cementation as dominant in this instance. The region's high sediment supply exerted a primary control on shelf preservation by dampening the erosive extent of wave-ravinement. Variations in antecedent gradient are responsible for much of the documented along-shelf and down-dip depositional variability: low-gradient settings promoted the development of broad coastal plains and strongly wave-dominated deltaic facies, while steeper areas of the shelf favoured the development of coastal landforms in relatively narrow complexes fronted by deep-water deltaic deposits. Steeper areas of the shelf also tend be characterised by an embayed along-coast profile that locally mitigates wave-energy, reinforcing the influence of inherited gradient.Once preserved, drowned coastal barriers introduced an additional geological control as post-glacial sediment became trapped between these features on the outer shelf. The dispersal of modern sediments is strongly influenced by the hydrodynamic regime Delagoa Bight, under which fluvial muds are transported northeastward along the shelf by a persistent inner-mid shelf counter-current. A considerable proportion of the modern sediment budget is lost off-shelf where the shelf narrows.This study shows that low antecedent gradients foster gentle coastal profiles predisposed to delta overstepping. Though local topographic knickpoints can aid in preserving the delta during submergence, they do not buffer the delta from the general effects of inundation which include drowning of the delta top. Once overstepped, the former delta can be overlain by distal deltaic sediments associated with the higher sea level and associated fluvial and oceanographic forcing quite different from earlier phases of delta construction.

Transgressive Architecture of Coastal Barrier Systems in the Ofanto Incised Valley and Its Surrounding Shelf in Response to Stepped Sea-Level Rise

Coastal deposits/barriers react to sea-level rise through rollover or overstepping. Preserved coastal deposits/barriers allow us to examine coastal responses to sea-level rise, an important aspect within the context of climate change. This study identifies the Ofanto incised valley and examines the possible factors that caused the considerable difference in shape between this valley and adjacent valleys: the Carapelle and Cervaro incised valley and Manfredonia incised valley. In addition, this study assesses the response of transgressive units to stepped sea-level rise with a focus on the evolution of palaeo-barriers/shorelines on the continental shelf and within the infill of Ofanto incised valley. We identified the traces of two slowstands in sea-level rise: the first, short-lived at a centennial scale, interrupted Meltwater Pulse 1A; the second is referable to part of Bølling-Allerød and Younger Dryas. During these two slowstands, two barrier-shoreface/estuarine-backbarrier systems formed. Meltwater Pulse 1A and Meltwater Pulse 1B led to overstepping and partial preservation of these systems in the form of aligned topographic highs. The second slowstand gave rise to continuous landward backstepping of the coastal barrier system; during the following Meltwater Pulse 1B (MWP-1B), landward rollover of the coeval barrier/backbarrier system occurred.

Siberian Permafrost Thawing Accelerated at the Bølling/Allerød and Preboreal Warm Periods During the Last Deglaciation

AbstractThis study investigated a continuous sediment core, retrieved from the deepest part in Lake Hovsgol, northwestern Mongolia. Its surrounds are occupied by a continuous Siberian permafrost zone. Distinct geochemical and lithological features suggest that the watershed soil was moistened during the last deglaciation, associated with climate amelioration. Especially, the calcareous glacial clay layers (ca 13.7 and ca 11.0 cal. ka BP), with high sulfur content peaks and positive shifts of δ34S values, well correlate with the timing of meltwater pulses 1A and 1B, leading to rapid warming such as Bølling/Allerød and Preboreal Holocene, respectively. This finding allows our interpretation for the calcareous glacial clay layers produced by landslide‐induced debris flow, which resulted from intensive water discharge from rapid thawing of permafrost during the abrupt warming periods. Our correlation with Lake Baikal records suggests that these permafrost thawings have accelerated over the area of the Selenga drainage basin (continental interior Eurasia).

Antecedent topography controls preservation of latest Pleistocene-Holocene transgression record and clinoform development: the case of the São Francisco delta (eastern Brazil)

The present study used shallow high-resolution seismic surveys to characterize the architecture and stratigraphic evolution of the last depositional sequence of the Sao Francisco Delta (SFDS). The sequence was accumulated within a bathymetric low (BL) on the shelf located in front of the Sao Francisco river. This antecedent topography provided additional accommodation of tens of meters in comparison with the rest of the shelf. The BL and its sediment-trapping effect allowed the accumulation of a complete stratigraphic record of the eustatic sea-level rise since the Last Glacial Maximum (LGM). The SFDS comprises six stratigraphic units. Because cores and radiocarbon dates are not available for this study area, the depth of occurrence of the top and base of each unit and their lateral relationships with wave-cut terraces carved into the BL walls were used in an effort to tentatively tie the mapped units to known eustatic sea-level curves available in the literature. Results suggest that major episodes of accumulation occurred during intervals of reduced rates in sea-level rise. The lower five units mapped accumulated in a large bay-estuarine protected environment, which resulted from the inundation of the BL. Only the topmost unit (the present clinoform) accumulated in an open ocean environment when sea level reached approximately its present position after 8 ka and the whole shelf was flooded. The most important delta backstepping event was probably associated with Meltwater Pulse 1B, which followed the Younger Dryas and produced one of the most significant stratal surfaces of the SFDS. Among the existing wave-dominated deltas in the Brazilian coast only, the Sao Francisco was able to construct a well-developed sigmoidal muddy clinoform in an otherwise very shallow shelf subjected to high wave-energy levels. This was only possible because of the existence of a bathymetric low in front of the river mouth, which illustrates the role that antecedent topography might play in providing larger amount of available accommodation space called in this paper of additional accommodation.

Late Quaternary sedimentological history of a submerged gravel barrier beach complex, southern Namibia

Through comprehensive seismic, stratigraphic, and sedimentological analysis, this paper describes the stratigraphic architecture of the late Quaternary sediments and depositional dynamics in a region ~ 3.5 km offshore of the current south-west Namibian coastline. The landscape evolution model of this area is based on 2D seismic reflection profiles calibrated to approximately 500 boreholes, which yielded sedimentary and seismic facies data of exceptionally high resolution. The data permitted the characterization of four sedimentary facies units and the reconstruction of the submerged landscape dominated by a barrier complex. We compare this barrier complex with the sediment bodies preserved along the adjacent coastline, and account for their geological evolution in response to late Quaternary relative sea-level fluctuations along the West Coast. The most noteworthy depositional units in the study area are the normal regressive coarse gravel beaches trending shore parallel, with the primary gravel barrier and its preserved coeval back-barrier deposits that exceed 7 km in length along the coast. Based on the radiocarbon dates from the rarely preserved early flooding facies (14.0 and 13.3 ka BP), the primary barrier complex formed during a stage of slowing rising relative sea-level subsequent to Meltwater Pulse 1A. Moreover, the barrier beach complex formed on a low-gradient palaeo-bathymetry slope that is punctuated by a break on its landward side (i.e., a regional knick point), which promoted the local accommodation of sediments. The preservation of the barrier beach complex and back-barrier deposits is attributed to rapid relative sea-level rise, linked to the early Holocene Meltwater Pulse 1B between ~ 11.5 and 11.2 ka BP, and an increase in back-barrier accommodation facilitated by the antecedent topography.

Permafrost-carbon mobilization in Beringia caused by deglacial meltwater runoff, sea-level rise and warming

During the last deglaciation (18–8 kyr BP), shelf flooding and warming presumably led to a large-scale decomposition of permafrost soils in the mid-to-high latitudes of the Northern Hemisphere. Microbial degradation of old organic matter released from the decomposing permafrost potentially contributed to the deglacial rise in atmospheric CO2 and also to the declining atmospheric radiocarbon contents (Δ14C). The significance of permafrost for the atmospheric carbon pool is not well understood as the timing of the carbon activation is poorly constrained by proxy data. Here, we trace the mobilization of organic matter from permafrost in the Pacific sector of Beringia over the last 22 kyr using mass-accumulation rates and radiocarbon signatures of terrigenous biomarkers in four sediment cores from the Bering Sea and the Northwest Pacific. We find that pronounced reworking and thus the vulnerability of old organic carbon to remineralization commenced during the early deglaciation (∼16.8 kyr BP) when meltwater runoff in the Yukon River intensified riverbank erosion of permafrost soils and fluvial discharge. Regional deglaciation in Alaska additionally mobilized significant fractions of fossil, petrogenic organic matter at this time. Permafrost decomposition across Beringia’s Pacific sector occurred in two major pulses that match the Bølling-Allerød and Preboreal warm spells and rapidly initiated within centuries. The carbon mobilization likely resulted from massive shelf flooding during meltwater pulses 1A (∼14.6 kyr BP) and 1B (∼11.5 kyr BP) followed by permafrost thaw in the hinterland. Our findings emphasize that coastal erosion was a major control to rapidly mobilize permafrost carbon along Beringia’s Pacific coast at ∼14.6 and ∼11.5 kyr BP implying that shelf flooding in Beringia may partly explain the centennial-scale rises in atmospheric CO2 at these times. Around 16.5 kyr BP, the mobilization of old terrigenous organic matter caused by meltwater-floods may have additionally contributed to increasing CO2 levels.

Controlling the external moment arm at the knee allows better recovery after multiple slipping perturbations during gait

Global reconstructions of the Earth's Quaternary ice sheet history such as ICE-6G assume that mantle rheology is linear and Earth properties are laterally homogeneous. However, these assumptions are unrealistic because high temperature/pressure creep experiments show that both linear and non-linear creep laws operate simultaneously in the mantle. Also surface geology and seismic tomography show that mantle properties vary laterally. This study uses the Coupled Laplace-Finite Element Method to find an ice model that is consistent with an Earth model with composite rheology where both linear and non-linear creep laws operate simultaneously in the mantle, and the effective viscosity is laterally heterogeneous.We use the ICE-6G ice model as initial load on a composite rheology Earth model and iteratively improve both the ice and Earth model parameters until they fit observations of Glacial Isostatic Adjustment or GIA (relative sea level and land uplift rate) well simultaneously. We thereby focus on North America and northern Europe. Our best combination model, called ICE-C(M_C), also fits gravity rate-of-change data in both areas. This finding removes the obstacle in using composite rheology in GIA studies since previous work found it impossible for composite rheology to fit both relative sea level and land uplift rate data simultaneously. It shows that composite rheology is clearly an alternative to linear rheology in future GIA studies.The ICE-C model is at the last glacial maximum (LGM) position from 26 ka BP (thousand years before present) to 19 ka BP, which implies that composite rheology also supports the timing of LGM proposed by Clark et al. (2009). The model is found to be significantly thicker in the center of rebound in Fennoscandia than ICE-6G from the LGM to about 8 ka BP, but significantly thinner around the Fennoscandian coastline from LGM to about 12 ka BP. These changes in the lower boundary conditions in northern Europe may affect the reconstruction of past climate there. Regional differences in ice thickness between ICE-C and ICE-6G are also found in Laurentia. During LGM there are negligible differences in total ice volume between ICE-C and ICE-6G, thus it remains questionable if a composite rheology may help in solving the missing ice problem. ICE-C is also consistent with meltwater pulses 1A and 1B. Mantle viscosities and creep parameters of Earth model M_C are consistent with previous findings from GIA modeling and microphysics experiments.

In search of an ice history that is consistent with composite rheology in Glacial Isostatic Adjustment modelling

Global reconstructions of the Earth's Quaternary ice sheet history such as ICE-6G assume that mantle rheology is linear and Earth properties are laterally homogeneous. However, these assumptions are unrealistic because high temperature/pressure creep experiments show that both linear and non-linear creep laws operate simultaneously in the mantle. Also surface geology and seismic tomography show that mantle properties vary laterally. This study uses the Coupled Laplace-Finite Element Method to find an ice model that is consistent with an Earth model with composite rheology where both linear and non-linear creep laws operate simultaneously in the mantle, and the effective viscosity is laterally heterogeneous.We use the ICE-6G ice model as initial load on a composite rheology Earth model and iteratively improve both the ice and Earth model parameters until they fit observations of Glacial Isostatic Adjustment or GIA (relative sea level and land uplift rate) well simultaneously. We thereby focus on North America and northern Europe. Our best combination model, called ICE-C(M_C), also fits gravity rate-of-change data in both areas. This finding removes the obstacle in using composite rheology in GIA studies since previous work found it impossible for composite rheology to fit both relative sea level and land uplift rate data simultaneously. It shows that composite rheology is clearly an alternative to linear rheology in future GIA studies.The ICE-C model is at the last glacial maximum (LGM) position from 26 ka BP (thousand years before present) to 19 kaBP, which implies that composite rheology also supports the timing of LGM proposed by Clark et al. (2009). The model is found to be significantly thicker in the center of rebound in Fennoscandia than ICE-6G from the LGM to about 8 kaBP, but significantly thinner around the Fennoscandian coastline from LGM to about 12 kaBP. These changes in the lower boundary conditions in northern Europe may affect the reconstruction of past climate there. Regional differences in ice thickness between ICE-C and ICE-6G are also found in Laurentia. During LGM there are negligible differences in total ice volume between ICE-C and ICE-6G, thus it remains questionable if a composite rheology may help in solving the missing ice problem. ICE-C is also consistent with meltwater pulses 1A and 1B. Mantle viscosities and creep parameters of Earth model M_C are consistent with previous findings from GIA modeling and microphysics experiments.

Sea-level oscillations in the East China Sea and their implications for global seawater redistribution during 14.0–10.0 kyr BP

The sea-level curve during the last deglaciation is characterized by two episodes of rapid sea-level rise, termed Meltwater Pulses (MWPs) 1A (14.6–14.3 kyr BP) and 1B (11.45–11.1 kyr BP), as well as an episode of relatively slow sea-level rise during the Younger Dryas stadial (YD; 12.85–11.65 kyr BP). However, the relative sea-level changes during the YD and MWP-1B periods are still not well defined. Here, we use precisely dated intertidal sediments derived from 6 sediment cores on the tectonically stable continental shelf of the East China Sea (ECS) to reconstruct a high-resolution sea-level curve between 14.0 kyr BP and 10.0 kyr BP. Although linear regression analysis suggests that the sea level rose continuously, with an average rate of 10.6 ± 0.6 mm/yr (r = −0.87, p(a) < 0.01, n = 95), the mode of sea-level rise differs before, during, and after the YD stadial. A rapid sea-level rise of 30.5 ± 4.9 mm/yr (r = −0.79, p(a) < 0.05, n = 25) occurred during 14.0–12.85 kyr BP. Subsequently, the rate of sea-level rise decreased significantly in response to the YD stadial, and it increased smoothly to 12.0 ± 1.4 mm/yr (r = −0.86, p(a) < 0.01, n = 29) from 11.65–10.0 kyr BP, indicating that MWP-1B was absent on the ECS shelf. Furthermore, based on comparisons of these data with other high-resolution curves of sea level or elevation at different latitudes, we suggest that the poleward seawater migration from intertropical broad oceans occurred because glacial isostatic adjustment could have generated a relatively consistent global sea-level height during the last deglaciation, especially during the YD, when the rate of sea-level rise was slowed.

Holocene sea-level history of the northern coast of South China Sea

This study has collected and analyzed seven sediment cores from the Pearl River delta, from which 16 new and high-quality sea-level index points are generated using a new approach. This study has also re-checked and re-calibrated the previously published sea-level data from China's southeast coast with corrections made for tectonic subsidence and sediment compaction factors. These sea-level data indicate a rise of relative sea level from −49.3 ± 0.8 m to the present height between 10,500 and 7000 cal. a BP. This sea-level history is similar to those recorded from other far-field locations and ice-volume equivalent sea-level models. The early to early-middle Holocene sea-level history in the study area shows a phase of accelerated rise at a rate increasing rapidly from 16.4 ± 6.1 mm/a at 10,500 cal. a BP to 33.0 ± 7.1 mm/a at 9500 cal. a BP. This phase was followed by a period of rapid decrease in the rate of sea-level rise to 8.8 ± 1.9 mm/a at 8500 cal. a BP and 1.7 ± 1.3 mm/a at 7500 cal. a BP. During the past 7000 years, the relative sea level in the study area changed very little. This new and complete history of Holocene sea-level change supports the following findings: (1) no obvious higher-than-present sea-level highstand in the Holocene is found from the northern South China Sea; (2) certain proportion of the effects of the predicted glacial isostatic adjustment were cancelled out by the effects of the weak upper mantle viscosity in the study area; (3) meltwater pulse 1b likely exists spanning into the early Holocene; (4) there are significant misfit between sea-level data and glacial isostatic adjustment models, and a revision to the existing ice melting history for the early Holocene is possibly needed.

The Açu Reef morphology, distribution, and inter reef sedimentation on the outer shelf of the NE Brazil equatorial margin

Submerged reefs, referred to as the Açu Reefs, have been newly observed on both sides of the Açu Incised Valley on the northeastern equatorial Brazilian outer shelf. This study aims to understand the roles of shelf physiography, its antecedent morphologies, and its inter reef sedimentation on the different development stages of the biogenic reef during last deglacial sea-level rise. The data sets consist of side-scan sonar imagery, one sparker seismic profile, 76 sediment samples, and underwater photography. Seven backscatter patterns (P1 to P7) were identified and associated with eleven sedimentary carbonate and siliciclastic facies. The inherited relief, the mouth of the paleo incised valley, and the interreef sediment distribution play major controls on the deglacial reef evolution. The reefs occur in a depth-limited 25–55 m water depth range and in a 6 km wide narrow zone of the outer shelf. The reefs crop out in a surface area over 100 km2 and occur as a series of NW-SE preferentially orientated ridges composed of three parallel ridge sets at 45, 35, and 25 m of water depth. The reefs form a series of individual, roughly linear ridges, tens of km in length, acting as barriers in addition to scattered reef mounds or knolls, averaging 4 m in height and grouped in small patches and aggregates. The reefs, currently limited at the transition between the photic and mesophotic zones, are thinly covered by red algae and scattered coral heads and sponges. Taking into account the established sea-level curves from the equatorial Brazilian northeastern shelf / Rochas Atoll and Barbados, the shelf physiography, and the shallow bedrock, the optimal conditions for reef development had to occur during a time interval (11–9 kyr BP) characterized by a slowdown of the outer shelf flooding, immediately following Meltwater Pulse-1B. This 2 kyr short interval provided unique conditions for remarkable reef backstepping into distinct parallel ridge sets. Furthermore, the Açu Reefs have trapped relict siliciclastic sediments within the three sets of reefs, west of the Açu Incised Valley and adjacent coasts. Lines evidence of easterly nearshore currents carried sediments from the old Açu Incised Valley and adjacent coasts. These incipiently drowned reefs influence the water circulation patterns of the modern shelf system, its carbonate sedimentation, and sediment transport. This study provides a new example of reef occurrence which might be more commonly observed on similar equatorial continental shelves.

Assimilating the ICE‐6G_C Reconstruction of the Latest Quaternary Ice Age Cycle Into Numerical Simulations of the Laurentide and Fennoscandian Ice Sheets

We analyze the effects of nudging 100 kyr numerical simulations of the Laurentide and Fennoscandian ice sheets toward the glacial isostatic adjustment‐based (GIA‐based) ICE‐6G_C reconstruction of the most recent ice age cycle. Starting with the ice physics approximations of the PISM ice sheet model and the SeaRISE simulation protocols, we incorporate nudging at characteristic time scales, τf, through anomalous mass balance terms in the ice mass conservation equation. As should be expected, these mass balances exhibit physically unrealistic details arising from pure GIA‐based reconstruction geometry when nudging is very strong (τf=20 years for North America), while weakly nudged (τf=1,000 years) solutions deviate from ICE‐6G_C sufficiently to degrade its observational fit quality. For reasonable intermediate time scales (τf=100 years and 200 years), we perturbatively analyze nudged ice dynamics as a superposition of “leading‐order smoothing” that diffuses ICE‐6G_C in a physically and observationally consistent manner and “higher‐order” deviations arising, for instance, from biases in the time dependence of surface climate boundary conditions. Based upon the relative deviations between respective nudged simulations in which these biases follow surface temperature from ice cores and eustatic sea level from marine sediment cores, we compute “ice core climate adjustments” that suggest how local paleoclimate observations may be applied to the systematic refinement of ICE‐6G_C. Our results are consistent with a growing body of evidence suggesting that the geographical origins of Meltwater Pulse 1B (MWP1b) may lie primarily in North America as opposed to Antarctica (as reconstructed in ICE‐6G_C).

Post-glacial flooding of the Bering Land Bridge dated to 11 cal ka BP based on new geophysical and sediment records

Abstract. The Bering Strait connects the Arctic and Pacific oceans and separates the North American and Asian landmasses. The presently shallow ( ∼ 53 m) strait was exposed during the sea level lowstand of the last glacial period, which permitted human migration across a land bridge today referred to as the Bering Land Bridge. Proxy studies (stable isotope composition of foraminifera, whale migration into the Arctic Ocean, mollusc and insect fossils and paleobotanical data) have suggested a range of ages for the Bering Strait reopening, mainly falling within the Younger Dryas stadial (12.9–11.7 cal ka BP). Here we provide new information on the deglacial and post-glacial evolution of the Arctic–Pacific connection through the Bering Strait based on analyses of geological and geophysical data from Herald Canyon, located north of the Bering Strait on the Chukchi Sea shelf region in the western Arctic Ocean. Our results suggest an initial opening at about 11 cal ka BP in the earliest Holocene, which is later than in several previous studies. Our key evidence is based on a well-dated core from Herald Canyon, in which a shift from a near-shore environment to a Pacific-influenced open marine setting at around 11 cal ka BP is observed. The shift corresponds to meltwater pulse 1b (MWP1b) and is interpreted to signify relatively rapid breaching of the Bering Strait and the submergence of the large Bering Land Bridge. Although the precise rates of sea level rise cannot be quantified, our new results suggest that the late deglacial sea level rise was rapid and occurred after the end of the Younger Dryas stadial.