mid-Holocene Sea-level Highstand Research Articles

Initiation of the Mekong River delta at 8 ka: evidence from the sedimentary succession in the Cambodian lowland

Modern deltas are understood to have initiated around 7.5–9 ka in response to the deceleration of sea-level rise. This episode of delta initiation is closely related to the last deglacial meltwater events and eustatic sea-level rises. The initial stage of the Mekong River delta, one of the world's largest deltas, is well recorded in Cambodian lowland sediments. This paper integrates analyses of sedimentary facies, diatom assemblages, and radiocarbon dates for three drill cores from the lowland to demonstrate Holocene sedimentary evolution in relation to sea-level changes. The cores are characterized by a tripartite succession: (1) aggrading flood plain to natural levee and tidal–fluvial channel during the postglacial sea-level rise (10–8.4 ka); (2) aggrading to prograding tidal flats and mangrove forests around and after the maximum flooding of the sea (8.4–6.3 ka); and (3) a prograding fluvial system on the delta plain (6.3 ka to the present). The maximum flooding of the sea occurred at 8.0 ± 0.1 ka, 2000 years before the mid-Holocene sea-level highstand, and tidal flats penetrated up to 20–50 km southeast of Phnom Penh after a period of abrupt ∼5 m sea-level rise at 8.5–8.4 ka. The delta progradation then initiated as a result of the sea-level stillstand at around 8–7.5 ka. Another rapid sea-level rise at 7.5–7 ka allowed thick mangrove peat to be widely deposited in the Cambodian lowland, and the peat accumulation endured until 6.3 ka. Since 6.3 ka, a fluvial system has characterized the delta plain, and the fluvial sediment discharge has contributed to rapid delta progradation. The uppermost part of the sedimentary succession, composed of flood plain to natural-levee sediments, reveals a sudden increase in sediment accumulation over the past 600–1000 years. This increase might reflect an increase in the sediment yield due to human activities in the upper to middle reaches of the Mekong, as with other Asian rivers.

Episodic intraplate deformation of stable continental margins: evidence from Late Neogene and Quaternary marine terraces, Cape Liptrap, Southeastern Australia

The Waratah Fault is a northeast trending, high angle, reverse fault in the Late Paleozoic Lachlan Fold Belt at Cape Liptrap on the Southeastern Australian Coast. It is susceptible to reactivation in the modern intraplate stress field in Southeast Australia and exhibits Late Pliocene to Late Pleistocene reactivation. Radiocarbon, optically stimulated luminescence (OSL), and cosmogenic radionuclide (CRN) dating of marine terraces on Cape Liptrap are used to constrain rates of displacement across the reactivated Waratah Fault. Six marine terraces, numbered Qt 6–Tt 1 (youngest to oldest), are well developed at Cape Liptrap with altitudes ranging from ∼1.5 m to ∼170 m amsl, respectively. On the lowest terrace, Qt 6, barnacles in wave-cut notches ∼1.5 m amsl, yielded a radiocarbon age of 6090–5880 Cal BP, and reflect the local mid-Holocene sea level highstand. Qt 5 yielded four OSL ages from scattered locations around the cape ranging from ∼80 ka to ∼130 ka. It formed during the Last Interglacial sea level highstand (MIS 5e) at ∼125 ka. Inner edge elevations (approximate paleo high tide line) for Qt 5 occur at distinctly different elevations on opposite sides of the Waratah Fault. Offsets of the inner edges across the fault range from 1.3 m to 5.1 m with displacement rates ranging from 0.01 mm/a to 0.04 mm/a. The most extensive terrace, Tt 4, yielded four Early Pleistocene cosmogenic radionuclide (CRN) ages: two apparent burial ages of 0.858 Ma ± 0.16 Ma and 1.25 Ma ± 0.265 Ma, and two apparent exposure ages of 1.071 Ma ± 0.071 Ma ( 10Be) and 0.798 Ma ± 0.066 Ma ( 26Al). Allowing for muonic production effects from insufficient burial depths, the depth corrected CRN burial ages are 1.8 Ma ± 0.56 Ma and 2.52 Ma ± 0.88 Ma, or Late Pliocene. A Late Pliocene age is our preferred age. Offsets of Tt 4 across the Waratah Fault range from a minimum of ∼20 m for terrace surface treads to a maximum of ∼70 m for terrace bedrock straths. Calculated displacement rates for Tt 4 range from 0.01 mm/a to 0.04 mm/a (using a Late Pliocene age, ∼2 Ma), identical to the rates calculated for the Last Interglacial terrace, Qt 5. This indicates that deformation at Cape Liptrap has been ongoing at similar time-averaged rates at least since the Late Pliocene. The upper terraces in the sequence, Tt 3 (∼110 m amsl), Tt 2 (∼140 m) and Tt 1 (∼180 m) are undated, but most likely correlate to sea level highstands in the Neogene. Terraces Tt1–Tt4 show an increasing northward tilt with age. The Waratah Fault forms a prominent structural boundary in the Lachlan Fold Belt discernible from airborne magnetic and bouger gravity anomalies. Seismicity and deformation are episodic. Episodic movement on the Waratah Fault may be coincident with sea level highstands since the Late Pliocene, possibly from increased loading and elevated pore pressure within the fault zone. This suggests that intervals between major seismic events could be on the order of 100 ka.

The mid-Holocene sea-level highstand at Bogenfels Pan on the southwest coast of Namibia

The radiocarbon ages of mollusc shells from the Bogenfels Pan on the hyper arid southern coast of Namibia provide constraints on the Holocene evolution of sea level and, in particular, the mid-Holocene highstand. The Bogenfels Pan was flooded to depths of 3 m above mean sea level (amsl) to form a large subtidal lagoon from 7300 to 6500 calibrated radiocarbon years before present (cal yr BP). The mollusc assemblage of the wave sheltered lagoon includes Nassarius plicatellus, Lutraria lutraria, and the bivalves Solen capensis and Gastrana matadoa, both of which no longer live along the wave-dominated southern Namibian coast. The radiocarbon ages of mollusc shell from a gravely beach deposit exposed in a diamond exploration trench indicate that sea level fell to near or 1 m below its present-day position between 6500 and 4900 cal yr BP. The rapid emergence of the pan between 6500 and 4900 cal yr BP exceeds that predicted by glacio-isostatic models and may indicate a 3-m eustatic lowering of sea level. The beach deposits at Bogenfels indicate that sea level rose to 1 m amsl between 4800 and 4600 cal yr BP and then fell briefly between 4600 and 4200 cal yr BP before returning to 1 m amsl. Since 4200 cal yr BP sea level has remained within one meter of the present-day level and the beach at Bogenfels has prograded seaward from the delayed arrival of sand by longshore drift from the Orange River. A 6200 cal yr BP coastal midden and a 600 cal yr BP midden 1.7 km from the coast indicate sporadic human utilization of the area. The results of this study are consistent with previous studies and help to refine the Holocene sea-level record for southern Africa.

Holocene valley aggradation driven by river mouth progradation: examples from Australia

AbstractSince the end of the post‐glacial sea level rise 6800 years ago, progradation of river mouths into estuaries has been a global phenomenon. The responses of upstream alluvial river reaches to this progradation have received little attention. Here, the links between river mouth progradation and Holocene valley aggradation are examined for the Macdonald and Tuross Rivers in south‐eastern Australia. Optical and radiocarbon dating of floodplain sediments indicates that since the mid‐Holocene sea level highstand 6800 years ago vertical floodplain aggradation along the two valleys has generally been consistent with the rate at which each river prograded into its estuary. This link between river mouth progradation and alluvial aggradation drove floodplain aggradation for many tens of kilometres upstream of the estuarine limits. Both rivers have abandoned their main Holocene floodplains over the last 2000 years and their channels have contracted. A regional shift to smaller floods is inferred to be responsible for this change, though a greater relative sea level fall experienced by the Macdonald River since the mid‐Holocene sea level highstand appears to have been an additional influence upon floodplain evolution in this valley. Copyright © 2006 John Wiley & Sons, Ltd.

Evidence of a Mid-Holocene Sea Level Highstand from the Sedimentary Record of a Macrotidal Barrier and Paleoestuary System in Northwestern Australia

This study addresses the mid- to late Holocene stratigraphy and sea level history of a macrotidal barrier and paleoestuary system located along the relatively unstudied northwest coast of Australia. Thirty-nine shallow cores were obtained from three transects perpendicular to the barrier and paleoestuary axis. Seven sedimentary facies were identified on the basis of sediment texture, carbonate content, and foraminifera assemblages: slope, upper intertidal mud, upper intertidal sand flat, lower intertidal sand flat, barrier, estuarine beach, and flood tide delta. The sedimentary infill reveals a fining upward succession of marine sediments up to 6 m thick, mostly along a regressive sequence. All facies are of Holocene age and started to be laid down when sea level was approximately 3 m below present elevation. A radiocarbon date from the topmost sedimentary facies (upper intertidal mud) indicates that relative sea level was at least 1 m higher than today by 2720 years BP. At this time, the estuary was at the final stages of sedimentary infill, with tidal inundation reduced to a minimum. Further evidence of a higher relative sea level during the Holocene is in the form of estuarine beach deposits found at the back of the paleoestuary at an elevation above the present day beach/dune interface. Net nearshore transport in the area, driven by tidal current asymmetry, is northward, and it is proposed that this has significantly influenced the alongshore component of the barrier progradation and evolution of the barrier estuary.

Vegetation change in the coastal-lowland rainforest at Avai'o'vuna Swamp, Vava'u, Kingdom of Tonga

Avai'o'vuna Swamp, a small coastal wetland in Vava'u, Kingdom of Tonga, produced a 4500-year pollen and sediment record. Results are: (1) a mid-Holocene sea level highstand is confirmed for Tonga between about 4500 and 2600 14C yr B.P.; marine clay contains pollen from mangroves ( Rhizophora mangle), coastal forest trees ( Barringtonia asiatica and Cocos nucifera), and rainforest trees ( Alphitonia, Rhus, Hedycarya and Calophyllum). (2) Microscopic charcoal first appeared at 2600 14C yr B.P., coincident with the arrival of Polynesians. (3) Cocos, Pandanus, Excoecaria, Macaranga, and Elaeocarpaceae pollen reflects the establishment of a mixed coastal-lowland rainforest in the last 2500 years. (4) The loss of Hedycarya, Elaeocarpus, Calophyllum, and Guettarda and the reduction of Terminalia and taxa in the Papilionaceae family by about 1000 years ago may be due to habitat destruction and the loss of dispersal capabilities of some species through the extinction of the two largest pigeons in Tonga.

On the Holocene sea-level highstand along the Yangtze Delta and Ningshao Plain, East China

The temporal-spatial distribution of Neolithic sites shows a long period of absence of human settlements during the early Holocene on the Yangtze Delta, and the area has not been colonized until 7000 aBP, corresponding to the onset of the Majiabang culture. Besides the regions east to the sandbar of Maqiao Site in Shanghai, foraminifera are not found in Neolithic cultural layers since 7000 aBP, indicating a widespread transgression between 10000 and 7000 aBP. Evidence of radiocarbon-dated cultural relics, foraminifera and pollen records from the Hemudu Site suggests that the Hemudu culture emerged and developed during the mid-Holocene Climatic Optimum. Therefore, the hypothesis of a mid-Holocene sea-level highstand along the Yangtze Delta and Ningshao Plain from 7000 to 5000 aBP is an open debate. New evidence from archaeological excavations and bio-stratigraphy suggests that, prior to 7000 aBP, a high sea level occurred along the Ningshao Plain. Subsequently, the sea level was lowered between 7000 and 5000 aBP, and the area suffered from frequent floods from 5000 to 3900 aBP.

Middle to late Holocene coastal evolution along the south coast of Upolu Island, Samoa

Stratigraphic surveys and sedimentological analyses of coastal sediments and reef cores along the south coast of Upolu Island, Samoa, reveal that during the middle Holocene this coast was characterised by barrier spits, open lagoons, and estuaries. These estuarine systems matured during the late Holocene, with progressive sedimentation and inlet closure, leading to the dominance of mangrove swamps in the past 1000 years. Contemporaneous with the transition of open estuaries to mangrove swamps was the aggradation and progradation of coastal plains. The coastal progradation since 700–1000 years BP is best explained by increased sediment availability and reduced incident wave energy at the shore resulting from the shallowing and subsequent cessation of reef crest accretion following the mid-Holocene sea-level highstand ca. ∼4500 yr BP. A small relative sea-level (RSL) lowering since 700–1000 years may have contributed to the positive sediment budget. This study highlights the need for island-wide coastal surveys to assess the relative roles of RSL, sediment budgets, and hydrodynamics on coastal evolution and stability. Differences in coastal evolution around Upolu Island may also be influenced by differential tectonic movements associated with late Holocene volcanism, coseismicity, and/or submarine landslides.

Mid-Holocene sea-level highstand along the Southeast Coast of China

Various sea-level curves have been proposed for the coast of China in the past two decades. These sea-level curves indicate a complex history of Holocene sea level, and so the debate on whether or not a higher mid-Holocene sea-level highstand exists in the coast of China has continued. This paper aims to re-examine the Holocene sea-level history for the low latitude part of the China coast (between 18°N and 32°N) by re-assessing all the sea-level data available from the east to south coasts and separating them according to geological settings in order to examine the influences of global and local factors. The reconstructed sea-level histories from different coastal sectors of the China coast reveal a certain degree of variability in the timing and height of mid-Holocene sea-level highstand. Within large river deltas, the mid-Holocene sea-level highstand occurred earlier by almost 1000 years than that from other coastal sites. The highstand from large river deltas appears also lower in altitude (a few metres below the present-day sea level) due probably to the local factors of subsidence and sediment consolidation. In geologically stable coastal sites, the highstand is recorded at the same altitude as the present-day sea level. A 1–2 m higher highstand is found from sites where tectonic uplift is observed.

Paleoshoreline record of relative Holocene sea levels on Pacific islands

Understanding the history of relative Holocene sea levels on Pacific islands is important for constraining fundamental geodynamic theories, interpreting the environments of early human occupation sites, and forecasting future environmental conditions on the islands. An observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels, hence standing at higher elevations than modern counterparts. Emergent paleoshoreline notches in limestone seacliffs record paleo-high-tide levels and emergent paleoreef flats record paleo-low-tide levels, whereas emergent paleobeachrock locally records paleo-intertidal levels. Both paleonotches and paleoreefs occur along the coasts of high-standing islands exposing volcanic bedrock and uplifted reef complexes, but low-lying coralline atolls lack sufficient relief to preserve paleonotches. Controls on relative Holocene sea level include global eustatic and regional hydro-isostatic changes in ambient sea level relative to island landmasses, and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism, by flexure of the lithosphere under the load of growing volcanic edifices (Hawaii, Samoa, Society Islands), by arching of the lithosphere over trench forebulges (Loyalty Islands, Niue, Bellona–Rennell), and by tectonism within forearc belts between active volcanic chains and trenches (Mariana Islands, Tonga, Vanuatu). The dominant pattern of relative sea-level change, where not overprinted by local tectonism or lithospheric flexure, was a uniform early Holocene rise in eustatic sea level followed by a regionally variable late Holocene hydro-isostatic drawdown in sea level. The resultant was a mid-Holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical Pacific Ocean. The earliest human migrations into intra-oceanic island groups of both the northwestern and southwestern Pacific Ocean followed close on the heels of the mid-Holocene sea-level highstand, and took advantage of newly attractive coastal environments engendered by sea-level drawdown. The effects of the mid-Holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence.

Holocene Sea-Level Record on Funafuti and Potential Impact of Global Warming on Central Pacific Atolls

Geomorphic features inherited from the mid-Holocene glacio-hydro-isostatic sea-level highstand that affected the central Pacific region influence the susceptibility of atoll islets to potentially enhanced wave erosion associated with rise in sea level from global warming. Shoreline morphology on multiple islets of Funafuti atoll in central Tuvalu reflects a relative mid-Holocene sea-level highstand 2.2–2.4 m above modern sea level. Typical islets are composed of unconsolidated post-mid-Holocene sediment resting disconformably on cemented coral rubble formed beneath now-emergent mid-Holocene reef flats. Exposed remnants of the lithified islet foundations serve as resistant buttresses protecting the flanks of atoll islets from wave attack. Islets lacking cemented mid-Holocene deposits as part of their internal structure are migratory sand cays with unstable shorelines. Any future sea-level rise ≥0.75 m, bringing high tide above the elevation of mid-Holocene low tide, might trigger enhanced wave erosion of stable atoll islets by overtopping the indurated mid-Holocene reef platforms. As analogous threshold relations are inferred for other central Pacific atolls, the risk of future inundation of island nations cannot be evaluated solely in terms of expected sea-level rise with respect to gross islet elevations.

9000 Years of sea-level change along the southern African coastline

Beachrocks are good indicators of sea-level stillstands on microtidal coastlines and data for this sea-level curve is mainly based on dating a series of Holocene beachrocks and planation episodes on the southeast African coastline. Sea-level reached its present level along the southern African coastline at ca. 6500 BP; sea-level then rose depositing a series of beachrocks at an elevation of + 2.75 m. This mid-Holocene sea-level highstand persisted for a period of 2500 years with a + 3.5 m stillstand, dated at 4480 BP, being the highest sea-level reached during the Holocene. In post-Holocene sea-level optimum times, a regression occurred lowering sea-level to its present level at 3880 BP and down to −2 m at approximately 3000 BP. Subsequently, sea-level rose to + 1.5 m at 1610 BP and attained its present level at about 900 BP. The mid-Holocene sea-level highstand appears to have resulted from a combination of isostatic emergence and the steric expansion of seawater relating to warmer ocean temperatures associated with the Holocene hypsithermal event on the eastern Agulhas Bank.