Sea-level Change Curve Research Articles

Astronomically forced paleoclimate and sea-level changes recorded in the continental margin of the South China Sea over the past ~23 m.y.

AbstractThe Earth has witnessed the emergence of continental-sized ice sheets, starting with Antarctica and gradually extending to both hemispheres over the past 40 million years. These ice accumulations have had a dramatic impact on both paleoclimate and sea level, substantially influencing sediment deposition in the continental margins. However, understanding sediment accumulation on an orbital scale in continental margins remains limited because of the scarcity of high-resolution, chronologically constrained sedimentary records. Here, we conducted a highly resolved cyclostratigraphic analysis based on natural gamma radiation (GR) series in depth domain at the continental margin of the South China Sea. We established a 22.8 m.y.-long high-resolution astronomical time scale spanning from the Miocene to the Quaternary by tuning the GR records to the global deep-sea benthic foraminifera carbon isotope curves and the 405 k.y. eccentricity cycles. The m.y.-scale sea-level changes since the Miocene were reconstructed through the sedimentary noise modeling of the 405-k.y.-tuned GR series. These reconstructions aligned with regional and global sea-level changes. The phase correlation between the filtered 1.2 m.y. cycles of sea-level change curves (dynamic noise after orbital tuning and ρ1 median models) from δ13Cbenthic and tuned GR series and the 1.2 m.y. obliquity amplitude modulation cycles revealed a shift from an anti-phase to an in-phase relationship across the middle Miocene climate transition (ca. 13.8 Ma), suggesting extensive expansion of the Antarctic ice sheet played a key role. In addition, a shift from an in-phase to an anti-phase relationship during the late Miocene (ca. 8 Ma and 5.3 Ma), indicating ephemeral expansion of the Arctic ice sheets or the changes in carbon cycle involving the terrestrial and deep ocean carbon reservoirs, might be the primary driver of eustatic changes. Furthermore, obliquity forcing and changes in meridional gradients in insolation that transported poleward flux of heat, moisture, and precipitation increased ice accumulation in both pole ice sheets and nonlinearly transferred high-latitude signals to low-latitude regions. This phenomenon is supported by the observation of strong obliquity signals in low latitude during global climate cooling interval. Our results suggest that m.y.-scale sea-level variations respond to astronomically induced climate change and ice sheet dynamics of both poles. This work contributes a highly resolved low-latitude geological archive to the future reconstruction of paleoclimate evolution on a global scale.

The Cambrian sequence filling process and sedimentary evolution of the Kuruktag area in the northeastern margin of the Tarim basin

The Kuruktag area in Xinjiang is located between the northeastern margin of the Tarim Basin and the South Tianshan orogenic belt, and its complex tectonic background has been extensively studied to assess the tectonic evolution of the Central Asian orogenic belt and the Tarim Craton. Although the Cambrian strata in the study area are well developed and continuous, due to the limitations of the outcrop environment and drilling and seismic data, studies of the sequence stratigraphy and sedimentary evolution have been limited and have yielded results with low accuracy, leading to limited knowledge of the local structure. In this paper, field observations and actual measurements of the stratigraphic profile, supplemented by perimeter logging and seismic data for stratigraphic sequence comparison, are collected, and sea level changes and controlling factors are discussed. Studies have shown that various types of sequence interfaces are present in the Kuruktag area, and three grade II and six grade III type-II sequences can be identified. The grade II and III sequences are composed of a transgressive systems tract (TST) and a highstand systems tract (HST), and the depositional thickness of the TST is less than that of the HST. The northern area has experienced an evolutionary process from a deep-sea basin to a deep-sea shelf, a shallow-water shelf, and a platform edge slope, while the southern area has evolved from a deep-sea basin to a deep-water shelf and shallow-water shelf before again transitioning to a deep-water shelf during the ЄSQ1-ЄSQ6 peroid. The sedimentary facies evolution and sequence structure indicate that the Kuruktag area has experienced a process of rapid large-magnitude transgression and slow regression in terms of sea level. In addition, the curve of sea level change for the Kuruktag area is quite different from the corresponding curves for the Tarim platform facies area and the North China platform. This difference is associated with the evolution of the Kuruktag rift basin.

Hidden hydrosphere under the Caspian Sea: Geophysical evidence and sea-level influence

We analyzed geologic-geophysical data, both from archives and collections in recent expeditions by the Shirshov Institute of Oceanology of Russian Academy of Sciences, and revealed many anomalies in the seismoacoustic wavefield. Anomalies are of two types: horizontal disturbed layers and vertical pipes. Anomalies form associations — pipes (chimneys) are rooted in disturbed layers and go through sediments up to the seafloor and form pockmarks. We consider all specified seismic anomalies and bottom microrelief as a possibility of vertical migration of gas and water via sediments (fluid from sediments to water and from seawater to sediments). It is expected that the fluids form a huge underground hydrosphere below the Caspian Sea. To test the fluid flow, we made a mathematical model of fluid discharge and absorption in bottom sediments. We hypothesize that the Caspian Sea level fluctuates, at least partially, due to cycles of the submarine ground water discharge and sea water absorption back into the sediments. This cyclicity of fluid discharge and absorption correlates with regional seismic events. The earthquakes cause tectonic relaxation, triggering the absorption process, and sea-level drop due to reverse flow into sediments. In other periods, the tectonic tension causes sediment compression and fluid discharge to sea water, which causes sea-level rise. The model was tested on the recent (past 100 years) sea-level change curve. As a result, we got a distribution of the general volume of fluid flows depending on the infiltration coefficient. The real change in the Caspian Sea volume is at the lower range of the calculated values, so we cannot neglect the effect of “gas pipes” (“chimneys”) on the Caspian Sea-level change.

Sedimentology and evolution of the Holocene radial tidal sand ridge in the south Yellow Sea, China

The radial tidal sand ridge (RTSR) off the Jiangsu Coast in the south Yellow Sea is one of the most fantastic and unique natural landscapes in the world. A large number of investigations on geomorphology and marine geology have been implemented during the last several decades. The formation and evolution of the RTSR, however, are still under debate. To resolve this issue, 10 ∼30–60 m-long cores were retrieved from the Jiangsu coastal plain and offshore sand ridge field in 2018–2019. Stratigraphic transects chronologically constrained by abundant radiocarbon dates were built based on sedimentological and paleoecological (i.e., foraminifer) data from selected cores. Combined with well-documented core stratigraphy from earlier studies, an age–depth plot with a local sea-level change curve revealed that the RTSR initiated approximately 9,000 cal a BP in a sandy bedform shape under transgressive conditions, continued throughout the Holocene, but developed mostly after 1128 AD, when the Yellow River flowed southward into the Yellow Sea. Abundant sediment supply, thus, appears to have played a major role in the formation and evolution of the RTSR in the south Yellow Sea.

Late Quaternary sedimentation and neotectonics in Liaodong Bay, northern Bohai Sea

The Bohai Sea is a typical shelf sea, with obvious sediment differences in different regions. Although Liaodong Bay is the largest bay in the Bohai Sea, there lacks boreholes with precise dating. In this paper, the core sediments taken from YKC-2 borehole in the northeastern of Liaodong Bay are dated with AMS14C and OSL, in addition to grain size analysis, foraminifera and pollen identification, lithological characteristics and other testing results. Correlation is made with the global sea level change curve, and upon the basis the evolution of paleoenvironment in the northeast of Liaodong Bay is reconstructed. It is speculated that the age of the bottom of borehole YKC-2 is 150 kyr B.P., and two transgressions have been recorded, similar to the situation in JXC-1, a borehole 46.7 km to the west. It can be found that the sedimentation rate of YKC-2 borehole is obviously higher than that of JXC-1. The authors’ research found that it is mainly controlled by two factors: the provenance difference and the differential movement of basin controlling faults. In the west, sediments are mainly transported by the Bohai Sea current, with insufficient sediment supply, and the basin-controlling faults are rather stable and very weak in activity. In the east, sediments are transported mainly by the surrounding rivers with sufficient sediment supply; and the changes in accommodation space are under the control of the Tan-Lu fault zone. Affected by the strong extensional dynamic environment of the Bohai Bay Basin, the accommodation space, where the borehole YKC-2 is located, continuously increased for deposition of near-source river sediments. This article is an endeavor to study the evolution of the Late Quaternary sedimentary environment and the influence of tectonic activity on Quaternary sedimentation in the Bohai Sea.

Marine geological researches in the greater bay of Navarino area, Southern Ionian sea

This paper examines the marine area of the Bay of Navarino and the Ionian sea sector to the west of the bay, based on a collection of shallow and intermediate penetration seismic profiles and on three gravity cores. The Bay has a smooth sea bottom morphology with maximum depth (75m) toward the south. Two minor reefs penetrate the sea bottom, one of them rising above the sea level forming a rocky island. In the open sea area the sea bottom is anomalous, especially after the shelf break, that is distinguished at a depth of approx. 160m. Beyond this depth deep valleys and ridges alternate and produce a rough morphology. The examination of the three gravity cores indicated that their sediment sand composition is mainly biogenic with minor amounts of terrigenous material. Increased shell fragments and terrigenous material was found at the bottom of one core taken at a depth of 116m, related probably to wave action at an earlier sea level position. The study of the seismic profiles collected within the bay and in the shelf area revealed,the presence of two distinct sedimentation units, un upper A filling with loose sediments the basins that are formed in the lower Y which is the basement. Within unit A two distinct subunits are recognized Al and A2, separated by an unconformity. In the sector to the west of the bay and beyond the shelf break, alternating opaque and transparent sediment packages are present, that are attributed to earlier sedimentation phases of low/high sea level stages. An older shelf break feature correlated with sea level stage 6 was distinguished at approx. 250m. sea depth. Thus subsidence rate in the area is estimated between 1 and 2 mm/year. The comparison of all data with the known sea level change curve indicated that unit AI was deposited during and after the latest transgression (stage 1) the unconformity between Al and A2 is related to the previous regression (stage 2), while the deposition of the unit A2 took place probably during stages 3 and/or 5. The morphology of the basement Y in the bay and the shelf area is thus attributed probably to stage 6 when the area was again subaerially exposed.

REKAMAN POSISI MUKA LAUT PADA AKHIR MASA DEGLASIAL DI PERAIRAN KEPULAUAN MATASIRI, LAUT JAWA

<strong>Sea Level Position Records of Last Deglaciation Era in Matasiri Islands Waters, Java Sea.</strong> During the Glacial Era, the Sunda Shelf was a dry land which then gradually inundated by sea water during Deglacial Era. This paper reveals four sea level positions in the Sunda Shelf at the end of Deglasial Era.The evidence of the sea level position was obtained from the findings of peat deposits in the four cores taken from the seabed of Matasiri Islands waters, South Kalimantan with gravity core method in the survey conducted in November 2010 by the research vessel Baruna Jaya VIII. Analysis of the position of the sea level was done by the depth of the current sea surface and the depth of peat deposits in the core. The results of the analysis of the sea level position showed four sea-level positions, i.e. -27.0, -41.3, -53.6, and -58.6 m below the current sea level. The aging analysis of sea level change curve based on the Sunda Shelf sequentially showing its age around 10, 11, 12, and 12.8 thousand years. The results of this study provided evidence that the track record relative changes in sea level during the end of Deglasial Era was also found in the Java Sea. <br /><br />

Mathematical modeling of Black Sea level change for the last 20000 years

A mathematical model of the Black Sea level change for the last 20,000 years is presented in this paper, based on the equations of viscous incompressible fluid motion according to Navier–Stokes, Sen-Venans, and the equation of water balance in the sea. The solution of the differential equations shows that the Black Sea level has been generally rising due to glacier melting and increase of freshwater balance in the late Pleistocene. Water which was flowing into the Black Sea did not have time to fully flow into the Marmara Sea because water was flowing through the long and very narrow canyon, which the Bosphorus strait was then. Water accumulating in the Black Sea increased the sea level, and increased the slope of the surface of river in the strait towards the Marmara Sea. When the level of the world ocean rose above river level, the strait began to form. Increasing depth caused excessive water to flow from the Black Sea. As a result of regression, the Black Sea level approached the Marmara Sea level, and the global oceanic transgression of the two seas began, continuing to the present.When the depth of the strait was 12–15 m, the penetration of the Marmara Sea's dense water into the Black Sea occurred. Water discharge in the counterflow was increasing when the ocean level was rising.The calculations showed that secondary fluctuations of the Black and Marmara Seas levels greater than 50 cm were theoretically impossible during the Holocene, as the deep strait allowed the possibility of intensive flow toward either the Black or Marmara Sea. When the value of the freshwater balance is positive, the regression of the Black Sea level below Marmara Sea level is theoretically impossible.Vertical movement of Earth crust can modify the shape of the eustatic sea level curves. A method allows calculation of the eustatic sea level course using the known local curves. The eustatic Mediterranean Sea level change and world ocean level change are calculated using nine local Mediterranean Sea level change curves. Tectonic components of the nine local curves were calculated. An assessment of the method's accuracy was made.

Facies analysis and sequence stratigraphy of the Asmari Formation in the northern area of Dezful Embayment, south-west Iran

This paper concerns the sequence stratigraphy of the Oligocene (Rupelian–Chattian)–Early Miocene (Burdigalian) Asmari Formation based on microfacies analysis of the sediments in the Ghale Nar Oilfield, central area of Zagros fold-thrust belt. Fourteen facies types typical for upward shallowing trend from open marine (MF 1–3), to shoal (MF 4-5), semi-restricted and restricted lagoon (MF 6–12) and finally to near-shore lagoon (MF 13-14) depositional environments were identified. Based on the environmental interpretations, we reconstructed a homoclinal ramp mainly represented by its inner and middle sectors. Facies types MF 4–14 are characterized by the occurrence of large and small porcelaneous benthic foraminifera representing a shallow-water setting of an inner ramp influenced by wave and tide processes. MF1 (Pabdeh Formation), and 2 -3 with planktonic, large and small hyaline benthic foraminifera represent between FWWB and SWB. Four third-order depositional sequences were recognized. In this study, in order to better correlate the identified sequences with pre-defined sequences of the Asmari Formation in the Dezful Embayment and the Izeh zone that are chronologically well-defined, we use their number instead of their name. Sequence 2 mainly consists of an open marine environment in the base, followed by semi-restricted to restricted lagoonal facies. Sequences 36 are characterized by semi-restricted to restricted lagoonal facies. Moreover, the regional relative sea-level change curves correlate well with the global sea-level change curves.

EVOLUTION OF SEDIMENTARY FACIES AND PALEOENVIRONMENT DURING THE LATE QUATERNARY IN WANQINGSHA AREA OF THE PEARL RIVER DELTA

Through the comprehensive analysis on chronostratigraphy,sedimentology,and paleoenvironment of core GZ-2 in Wanqingsha area of the Pearl River delta,it is suggested that the Holocene sedimentation began from the depth of -15.37 m (about 6 kaBP),and strata below this level to the depth of -40.35 m belong to middle-late late Pleistocene with a maximum dating value around 43 kaBP,contemporaneous with the earliest deposit in the Pearl River delta. In the profile,two cycles of transgression-regression can be recognized:the middle-late late Pleistocene(consisting of 4 sub-cycles ) and the Holocene(consisting of 2 sub-cycles). It still has,in this area,fluvial-lake deposition facies during the lower sea level period of the Last Glaciation. The reconsruction of relative sea-level change curve of the Pearl River delta based on the multi-proxy index analysis indicates that the highest sea level period of the middle-late late Pleistocene was around 26～24 kaBP,and that of the Holocene was around 6～2.5 kaBP. The late Quaternary paleoenvironment evolution in the Pearl River delta was obviously controlled by global paleoenvironment changes.

The Asmari Formation, north of the Gachsaran (Dill anticline), southwest Iran: facies analysis, depositional environments and sequence stratigraphy

The Asmari Formation is exposed at the Dill anticline in the Dezful Embayment of the Zagros foreland basin with 206 m thickness comprising thin, medium and thick to massive bedded carbonates. The Asmari Formation is Late Oligocene (Chattian)–Early Miocene (Burdigalian) in age at the study area. Ten facies characterizing upward gradual shallowing trend of an open marine (MF 1–3), shoal (MF 4), semi-restricted and restricted lagoon (MF 5–9) and near-shore lagoon (MF 10) depositional environments were identified. Based on environmental interpretations, a homoclinal ramp consisting of inner and middle parts prevails. MF 4–10 were characterized by the occurrence of large and small porcelaneous benthic foraminifera representing a shallow-water setting of an inner ramp influenced by wave and tide processes. MF 1–MF 3 with large and small hyaline benthic foraminifera represent a deeper fair water wave base of a middle ramp setting. Three-third-order depositional sequences were recognized. Sequence 1 mostly consists of an open marine to the lower part and is followed by semi to restricted lagoon facies. Sequences 2 and 3 are characterized by semi to restricted lagoon facies. Moreover, the relative sea-level change curves correlate with the global sea-level change curves.

Growth Normal Faulting at the Western Edge of the Metropolitan Taipei Basin since the Last Glacial Maximum, Northern Taiwan

Growth strata analysis is an useful tool in understanding kinematics and the evolution of active faults as well as the close relationship between sedimentation and tectonics. Here we present the Shanchiao Fault as a case study which is an active normal fault responsible for the formation of the 700-m-thick late Quaternary deposits in Taipei Basin at the northern tip of the Taiwan mountain belt. We compiled a sedimentary record, particularly the depositional facies and their dated ages, at three boreholes (SCF-1, SCF-2 and WK-1, from west to east) along the Wuku Profile that traverses the Shanchiao Fault at its central segment. By incorporating the global sea level change curve, we find that thickness changes of sediments and changes of depositional environments in the Wuku area are in a good agreement with a rapid sea level rise since the Last Glacial Maximum (LGM) of about 23 ka. Combining depositional facies changes and their ages with their thickness, we are able to introduce a simple back-stripping method to reconstruct the evolution of growing strata across the Shanchiao Fault since the LGM. We then estimate the vertical tectonic slip rate since 23 ka, which exhibits 2.2 mm yr-1 between SCF-2 and WK-1 and 1.1 mm yr-1 between SCF-1 and SCF-2. We also obtain the Holocene tectonic subsidence rate of 2.3 mm yr-1 at WK-1 and 0.9 mm yr-1 at SCF-2 since 8.4 ka. We thus conclude that the fault zone consists of a high-angle main fault to the east between SCF-2 and WK-1 and a western lower-angle branch fault between SCF-1 and SCF-2, resembling a tulip structure developed under sinistral transtensional tectonism. We find that a short period of 600-yr time span in 9 8.4 ka shows important tectonic subsidence of 7.4 and 3.3 m for the main and branch fault, respectively, consistent with possible earthquake events proposed by previous studies during that time. A correlation between geomorphology and subsurface geology in the Shanchiao Fault zone shows that an array of subtle geomorphic scarps corresponds to the branch fault, however, that the surface trace of the main fault seems to be completely erased by the surface force of erosion and sedimentation. We recommend that analysis in the light of growth-faulting scheme should be conducted in a more systematic way for earthquake geology study in other portions of the Shanchiao Fault within the Taipei Basin, as well as other areas worldwide currently under extensional deformation.

Carbonate Sequence Stratigraphy of a Back‐Arc Basin: A Case Study of the Qom Formation in the Kashan Area, Central Iran

Abstract: The Qom Formation comprises Oligo‐Miocene deposits from a marine succession distributed in the Central Basin of Iran. It is composed of five members designated as A‐F. Little previous work exists on the sequence stratigraphy. Based on an integrated study of sequence stratigraphy with outcrop data, wells and regional seismic profiles, the Qom Formation is interpreted as a carbonate succession deposited in a mid‐Tertiary back‐arc basin. There are two second‐order sequences (designated as SS1 and SS2) and five third‐order sequences (designated as S1‐S5). Five distinct systems tracts including transgressive, highstand, forced regressive, slope margin and lowstand have been recognized. The relationship between the sequences and lithologic sub‐units has been collated and defined (S1 to S5 individually corresponding to A‐C1, C2‐C4, D‐E, the lower and upper portions of F); a relative sea level change curve and the sequence stratigraphic framework have been established and described in detail. The coincidence of relative sea level change between that of the determined back‐arc basin and the world indicates that the sedimentary cycles of the Qom Formation are mainly controlled by eustatic cycles. The variable combination of the systems tracts and special tectonic‐depositional setting causally underpin multiple sequence stratigraphic framework styles seen in the carbonates of the back‐arc basin revealing: (1) a continental margin basin that developed some form of barrier, characterized by the development of multiple cycles of carbonate‐evaporites; (2) a flat carbonate ramp, which occurred on the southern shelf formed by the lack of clastic supply from nearby magmatic islands plus mixed siliciclastics and carbonates that occurred on the northern shelf due to a sufficient clastics supply from the land; and (3) a forced regressive stratigraphic stacking pattern that occured on the southern shelf and in basin lows due to the uplifting of the southern shelf. Thick and widespread aggradational framework limestone usually occurs in the initial sequences (S1 and S3) of the supersequence, which led to preferential oil reservoir deposition but a lack of source and cap rocks, whereas the retrogradational and progradational framework limestone usually occurs in the later sequences (S2 and S4‐S5) of the supersequence, which results in two perfect sets of source, reservoir and cap rock assemblies, so that the limestone in sub‐member C2‐C4 and the F‐Member can be predicted as important objects for oil exploration.

Long-term effect of the Kärdla crater (Hiiumaa, Estonia) on Late Ordovician carbonate sedimentation

Abstract Kardla impact crater, 4 km in diameter, was formed in the early Caradoc (Late Ordovician) in the Baltoscandian shelf sea, a sediment-starved temperate-water carbonate basin. The Upper Ordovician post-impact sediments that cover the Kardla crater are 15–275 m thick. The crater rimwall is composed of three separate basement rises that formed islands or shoals in the Late Ordovician shelf sea and influenced the sedimentation in the surrounding area. The rises provide the means to study the shallow-water sedimentary environments and sea-level history. This may be one of the few cases, where shore environments of the Baltoscandian Ordovician palaeobasin are preserved in the geological record. Lithofacies distribution was studied in the crater area from data of about 100 drillcores. Grain size distribution and composition of the non-carbonate material were analyzed in six core sections in order to reconstruct the history of crater erosion. The post-impact marine deposits lie on a partly redeposited and mixed ejecta layer derived from the Cambrian to Lower Ordovician silici-clastic deposits in the target area. Beds of marine carbonate sediments of Caradoc age, wackestones and mudstones, on-lap on the slope of the crater rim. A shift of sedimentation towards the crater rim during the Caradoc is evidence of a relative sea-level rise, which was a combination of early Caradoc eustatic rise, crater area subsidence, and compaction of pre-impact and impact sediments. The occurrences of coarse sand and gravel in specific stratigraphic intervals reflect the intensity of wave action and sea-level changes in the area. A detailed curve of sea-level changes during Late Ordovician has been constructed and different post-impact sedimentation models are presented for the crater area. The sedimentation pattern changed during the Caradoc as a result of sea-level rise and climatic change. The synchronous appearance of echinoderm bioclastic accumulations in shoal areas in Estonian mainland and in the Kardla rim dates the climatic change in the basin area as late Keila (middle Caradoc) time. Assuming that the Caradoc lasted about 11 million years, infilling of the Kardla crater required about 7 million years, and influence of the crater structure on sedimentation was obvious even 10 million years after the impact event. The tectonic stability of the region, the low rate of sedimentation, and long lasting differential compaction of sediments in the crater area account for this extraordinary long influence of crater. The Kardla crater is an example of the astrobleme, formed in ancient shelf sea, which may be used to reconstruct basin history of post-impact times.

Le passage Ordovicien-Silurien à Tizi n'Tichka (Haut-Atlas, Maroc) : variations du niveau marin

In the Tizi n'Tichka area, the Late Ordovician sedimentation is characterized by two depositional sequences of glacio-eustatic origin: S1 and S2. Two other post-glacial sequences are proposed for the Lower Silurian deposits: at the Llandovery (S3) and at the Sheinwoodian (S4) ages. Their sea-level change curve is compared with curves from other continents.

Strontium isotope stratigraphy of Kita-daito-jima Atoll, North Philippine Sea: implications for Neogene sea-level change and tectonic history

A detailed chronology of a 432 m core from Kita-daito-jima, an atoll on the Philippine Sea plate, has been obtained by strontium isotope stratigraphy. The atoll carbonate extends to late Oligocene age. The top ∼ 100 m Plio-Pleistocene section of the core has been dolomitized, and dolomite also extends to middle Miocene sediments. Sr isotopes identify two dolomitization events, at ∼ 2 and ∼ 5 Ma. Atoll growth appears to have been continuous between 18.8 and 24.3 Ma and the fit of the growth history to a standard model provides a successful confirmation of Darwin's subsidence theory. The atoll growth appears to have commenced about 42 Ma ago (6 Ma after volcanism), producing a ∼ 2 km column of atoll carbonate, in agreement with bathymetry. The subsidence model defines an apparent loss of 360 m of the sedimentary record since the early-middle Miocene boundary. The amount in excess of the ∼ 100 m associated with sea-level fall is attributed to Plio-Pleistocene uplift at rates of ∼ 20–30 m/Ma associated with the forebulge of the Philippine Sea plate prior to its subduction. Uplift rates have also been estimated using (a) the heights of dated last-interglacial coral reefs on Kita-daito-jima and nearby Minami-daito-jima (∼ 17 m/Ma and ∼ 29 m/Ma), (b) Sr isotope ages of calcites from surface samples on Minami-daito-jima (∼ 21 m/Ma) and (c) contrasts in elevation with Okino-daito-jima 100 km away (∼ 22 m/Ma). The locations of hiatuses and ages of dolomitization indicate sea-level falls of ∼ 80 m at ∼ 17–16 Ma, ∼ 30 m at ∼ 16–15 Ma, ∼ 125 m at ∼ 11 Ma, and ∼ 90 m at ∼ 5 Ma and at ∼ 2 Ma. The data have been combined with those from Enewetak Atoll to produce a curve of sea-level change for the Neogene.

千葉県古流山湾周辺域における完新世の環境変遷史とその意義

The continuing history of paleoenvironments in the Holocene was investigated in a small valley, the Sakagawa lowland (Paleo-Nagareyama Bay), situated in Nagareyama City, north of Tokyo. In this area, the seawater of the Jomon transgression invaded the valley, which had been deeply dissected during the last glaciation. 14C dating, tephra identification, grain-component analysis, diatom analysis, and pollen analysis showed that the sediments recorded some important evidence of paleogeographic changes, especially in sea-level and vegetation.As a result of the comparison in vertical changes among some fossil assemblages, the ages 6, 500, 5, 300, 4, 500, and 3, 500y.B.P. were recognized as common and important boundaries in Holocene environmental history. Moreover, such ages correspond to the turning points in the relative sea-level change curve which was drawn based on 65 14C dates obtained in the central Kanto Plain.The Holocene environmental history of the area is as follows:1) 7, 500-6, 500y.B.P.: Sea-level rose rapidly and a deep inner bay formed. On terraces surrounding the bay, the warm-temperate broad-leaved deciduous forest mainly composed of Quercus (Lepidobalanus) was dominant, along with lucidophyllous tress such as Castanopsis and Quercus (Cyclobalanopsis).2) 6, 500-5, 300y.B.P.: Sea-level was stable at +3m, the tidal flat developed, and oyster reefs were formed over a wide area.3) 5, 300-4, 500y.B.P.: About 5, 300y.B.P., sea-level was abruptly lowered to +2-+1m, and sand layers covered the reef. Those sandy materials were transported not only from the surrounding terraces, but also from the Paleo-Okutokyo bay, the major body of water in the area. The broad-leaved deciduous forest continued to be dominant on the terraces.4) 4, 500-3, 500y.B.P.: The sea-level was stable again, and oyster reefs were formed. Marshes accompanied by swamp forests composed of Alnus covered the sea area. Lucidophyllous forest, mainly of Quercus (Cyclobalanopsis), because dominant.5) 3, 500y.B.P.-Present: The sea area regressed gradually from 3, 500y.B.P. to 1, 800y.B.P. due to the lowering of the sea-level to -1--2m. During the early half of this stage, lucidophyllous forest was dominant on the terraces, and Alnus swamp forest developed in the valley. Alnus swamp forest, however, disappeared suddenly at 1, 800y.B.P., when the sea-level rose to 0m. After 1, 800y.B.P., Cryptomeria and aquatic plant pollen increased dramatically.

完新世における対馬暖流の脈動

Surface water temperature during the Holocene, estimated from diatoms in two pistoncores-KH-79-3, L-3 from the southern part of the Japan Sea and KH-84-3, St. 33 from the western part of the Tsugaru Strait-shows good correlation with pollen flora, oxygen isotope, and sea level change curves on a time scale of 103 years. Rhythmic fluctuations of diatom temperature (Td=Xw/(Xw+Xc)) values recognized in both pistoncores indicate the extent of influx of the Tsushima warm current into the Japan Sea. Four peaks of Td values, indicating the strong influx of the Tsushima warm current (warm period), occur within a interval from the Akahoya ash (14C date, 6, 300Y.B.P.) at 86-94cm level to 0cm level in piston-core L-3.

Post-Triassic continental hypsometry and sea level

Application of modern hypsometric relationships to ancient continental groupings requires changes in ocean basin volumes through time and hence sea level. A global sea-level change curve, based on current knowledge of continental evolution from the base of the Jurassic, is presented. Variation in sea level for most of the Mesozoic, requiring explanation by mechanisms other than the area–elevation effect, was of the order of 160 m; a major lowering of sea level started in the late Cretaceous and continued into the Cenozoic. This major change can be explained by changes in ocean ridge volumes.

Sea-level changes in northern England

The terms transgressive and regressive overlap current in solid rock geology since the early nineteenth century are applied as descriptive terms to stratigraphic successions in northern England. Two methods of displaying radiocarbon dates from overlap sequences are described and a chronostratigraphic subdivision of marine and terrestrial sediments is proposed: 12 periods, during which transgressive overlap occurred, and 12 periods, during which regressive overlap occurred, are recognised, and supercede the transgression sequences Lytham I to IX. Two new time-depth graphs are described from southwest Lancashire and the Morecambe Bay area, and each radiocarbon date is shown in relationship to its position in a transgressive or regressive overlap. Tentative interpretations are made about the form of the curve of sea-level changes.