R. K. Matthews

Quaternary sea level fluctuations on a tectonic coast: New 230Th234U dates from the Huon Peninsula, New Guinea

Emerged coral reef terraces on the Huon Peninsula in New Guinea were reported in a reconnaissance dating study by Veeh and Chappell 1970. Age definition achieved was not good for several important terraces, and we report here a series of new 230Th234U dates, which further clarify the history of late Quaternary eustatic sea level fluctuations. More than 20 reef complexes are present, ranging well beyond 250,000 yr old: we are concerned with the seven lowest complexes. Major reef-building episodes dated by 30Th234U are reef complex I at 5–9 ka (kilo anno = 1000 yr), r.c. IIIb at 41 ka (four dates), r.c. IV at 61 ka (four dates), r.c. V at 85 ka (two dates), r.c. VI at 107 ka (two dates), and r.c. VII at 118–142 ka. Complex II was previously dated by 14C at 29 ka: this age has not yet been confirmed, and may be only a lower limit. The reef crests were built during or immediately before intervals of sea level maxima, when rates of rising sea level and tectonic uplift briefly coincided. The culmination of each reef-building episode was only a few thousand years in duration, and multiple dates from the same reef complex generally group within the statistical errors of the individual dates. Several methods can be used to estimate the altitude of each sea level maximum relative to present sea level. The least complicated is to calculate mean tectonic uplift rate for each profile of the terraces, and use the mean rate to calculate the tectonic displacement of each dated reef complex on that profile. The difference between the present altitude of a reef complex and its calculated tectonic uplift gives the paleosea level at the time the reef grew. We estimate uplift rates for six surveyed sections by calibrating against published paleosea level estimates from Barbados and elsewhere, viz 125 ka, paleosea at +6 m; 103 ka, −15 m; 82 ka, −13 m. For each section the individual uplift rates for reefs V, VI, and VIIb are within 5% of their section means. Using the mean rates. paleosea level estimates for reef crests II, IIIB, and IV are made for each section. Consistency of estimates between sections is good, giving −28 m for the 60 ka paleosea level, around −38 m for the 42 ka level and −41 m for the 28 ka level (if the age is older the paleosea level would be lower. Using the mean uplift rates, the 82 ka and 103 ka paleosea levels are also estimated for each section: all individual estimates are plotted graphically, and a sea level curve drawn. The reef stratigraphy indicates sea level lowerings between each dated reef crest: the crests probably represent the interstadials of the Wisconsin (Wurm, Weichsel) Glaciation, and intervening lower levels correspond to stadials. Since the last time of eustatic sea level higher than the present (about 125 ka), five sea level maxima occurred at roughly 20-ka intervals, none being as high as the present.

Milankovitch Hypothesis Supported by Precise Dating of Coral Reefs and Deep-Sea Sediments

Barbados provides a possibly unique opportunity for reconstruction of the times and elevations of late-Pleistocene high stands of the sea. The island appears to be rising from the sea at a uniform rate that is fast enough to separate in elevation coral-reef tracts formed at successive high stands of the sea. Unaltered coral found in the lower terraces enables high-precision Th230: U234 and Pa231: U235 dating. Three distinct high stands of the sea are found about 122,000, 103,000, and 82,000 years ago. New Pa231 and Th230 dates from a deep-sea core also indicate that Ericsons W-X cold-to-warm climatic change occurred close to 126,000 years ago. These data show a parallelism over the last 150,000 years between changes in Earths climate and changes in the summer insolation predicted from cycles in the tilt and precession of Earths axis.

The last interglacial ocean

The final effort of the CLIMAP project was a study of the last interglaciation, a time of minimum ice volume some 122,000 yr ago coincident with the Substage 5e oxygen isotopic minimum. Based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean, last interglacial sea-surface temperatures (SST) were compared with those today. There are small SST departures in the mid-latitude North Atlantic (warmer) and the Gulf of Mexico (cooler). The eastern boundary currents of the South Atlantic and Pacific oceans are marked by large SST anomalies in individual cores, but their interpretations are precluded by no-analog problems and by discordancies among estimates from different biotic groups. In general, the last interglacial ocean was not significantly different from the modern ocean. The relative sequencing of ice decay versus oceanic warming on the Stage 6/5 oxygen isotopic transition and of ice growth versus oceanic cooling on the Stage 5e/5d transition was also studied. In most of the Southern Hemisphere, the oceanic response marked by the biotic census counts preceded (led) the global ice-volume response marked by the oxygen-isotope signal by several thousand years. The reverse pattern is evident in the North Atlantic Ocean and the Gulf of Mexico, where the oceanic response lagged that of global ice volume by several thousand years. As a result, the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years. These regional lead-lag relationships agree with those observed on other transitions and in long-term phase relationships; they cannot be explained simply as artifacts of bioturbational translations of the original signals.

13C Record of benthic foraminifera in the last interglacial ocean: Implications for the carbon cycle and the global deep water circulation

The 13C/12C ratios of Upper Holocene benthic foraminiferal tests (genera Cibicides and Uvigerina) of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide (TCO2) measured during the GEOSECS program. The δ13C difference between benthic foraminifera and TCO2 is 0.07 ± 0.04‰ for Cibicides and −0.83 ± 0.07‰ for Uvigerina at the 95% confidence level. δ13C analyses of the benthic foraminifera that lived during the last interglaciation (isotopic substage 5e, about 120,000 yr ago) show that the bulk of the TCO2 in the world ocean had a δ13C value 0.15 ± 0.12‰ lower than the modern one at the 95% confidence level, reflecting a depletion, compared to the present value, of the global organic carbon reservoir. Regional differences in δ13C between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one: the Antarctic Bottom Water production was higher than today during the last interglaciation, but the eastward transport in the Circumpolar Deep Water was lower.

The marine oxygen isotope record in Pleistocene coral, Barbados, West Indies☆

The reef-crest coral Acropora palmata from late Pleistocene reefs on Barbados has recorded the same global variations in oxygen isotopes as planktonic and benthonic foraminifera. Although the record of oxygen isotopes in Acropora palmata is discontinuous, it offers several advantages over the isotope records from deep-sea sediments: (1) the coral grows at water depths of less than 5 m; (2) the samples are unmixed; (3) specimens may be sampled from various elevations of paleo-sea level; and (4) aragonitic corals are suitable for 230Th234U and HeU dating techniques. The latter advantage means that direct dating of the marine oxygen isotope record is possible. Oxygen isotope stage 5e corresponds to Barbados III, dated at 125,000 ± 6000 yr BP. Petrographic and geochemical evidence from five boreholes drilled into the south coast of Barbados indicates a major eustatic lowering (greater than 100 m below present sea level) occurred between 180,000 and 125,000 yr BP. The age and isotopic data suggest correlation of this change in sea level to Emilianis oxygen isotope stage 6. Acropora palmata deposited at various elevations of sea level during oxygen isotope stage 6 vary by 0.11 ‰ δ18O for each 10 m of change in sea level. We further hypothesize a minimum drop of 2°C in the average temperature occurred during the regressive phase of oxygen isotope stage 6. These data indicate that temperature lowering of surface water near Barbados lagged behind a major glacial buildup during this time period. Using the δ18O vs sea level calibration herein derived, we estimate the relative height of sea stands responsible for Barbados coral reef terraces in the time range 80,000 to 220,000 yr BP.

Uranium-series dating of the Pleistocene reef tracts of Barbados, West Indies

Detailed studies of reef-tract stratigraphy in the southwestern part of Barbados have revealed nine, seven, and ten reef tracts in the Christ Church, Clermont Nose, and Saint George9s Valley sections, respectively. The reefs have been projected onto standard traverses, and present elevations are reported for each reef crest. To date the reef crests by Th 230 /U and He 4 /U methods, the U 234 /U 238 and Th 230 /U 234 activity ratios and U and He 4 concentrations were determined in about 35 unrecrystallized coral samples. Discordant U 234 /U 238 , Th 230 /U, and He 4 /U ages for many samples indicate that both U 234 and Th 230 have been diagenetically added to the samples. He 4 /U ages are corrected by subtracting the He 4 added due to the presence of “open system” U-series nuclides; the correction ranges from 0% to 10%. The reef-tract ages and present elevations as projected on the standard traverse show that prior to 125,000 B.P., the uplift rate of the Christ Church section was much greater than the rate after 125,000 B.P. Paleo-sea levels inferred from Saint George9s Valley reef tract ages and elevations cluster about the present datum, which suggests that throughout the Brunhes epoch, minimum interglacial continental ice volume was comparable to the present value. Within the uncertainty of the age and paleo—sea-level estimates, the Brunhes sea-level history inferred from Barbados reef-tract chronostratigraphy is consistent with that inferred by Shackleton and Opdyke from the oxygen isotope record of core V28-238.

Tertiary δ18O record and glacio-eustatic sea-level fluctuations

Previous interpretation of the Tertiary δ 18 O record of plaiiktic and benthic foraminifers has emphasized comparison to the modern ocean, assumed an ice-free world prior to middle Miocene time, and thereby calculated surprisingly cool temperatures for the tropical sea surface. We propose an alternative interpretation, which compares Tertiary data to average late Pleistocene, assumes constant tropical sea-surface temperature, and thereby estimates global ice volume. This approach suggests that Earth has had a significant ice budget (and therefore glacio-eustatic sea-level fluctuations) at least since Eocene and perhaps even throughout much of Cretaceous time.

Cenozoic ice-volume history: Development of a composite oxygen isotope record

To view Cenozoic ice-volume and ocean-temperature history, a composite δ 18 O time series was constructed for tropical shallow-dwelling planktonic foraminifers and for deep-water benthic foraminifers. We conclude that Cenozoic benthic δ 18 O primarily reflect considerable deep-ocean temperature variation and that tropical planktonic δ 18 O afford the best record of Cenozoic ice volume. The planktonic composite indicates the presence of a significant ice budget for the past 40 m.y. The difference between the benthic and planktonic δ 18 O composites suggests that low-latitude production of warm saline bottom water dominated Tertiary thermohaline circulation.

Carbon and oxygen isotopes as diagenetic and stratigraphic tools: Surface and subsurface data, Barbados, West Indies

Carbon isotopic values reported in the literature for low-magnesium calcite Pleistocene limestones are generally more negative than −4‰ PDB, while values reported for most ancient limestones are more positive than −2‰ PDB. This relationship has recently led to suggestions that subaerial diagenesis was of minor importance in forming ancient limestones. Isotopic analyses of Barbados subsurface Pleistocene limestone samples show that samples from shallow subsurface diagenetic regimes have δC13 compositions much closer to those reported for ancient rocks. In the vadose environment, Barbados limestones become systematically enriched in C13 by 3 to 4‰ with increasing depth. This δC13 versus depth relationship may prove useful for identifying subaerial exposure surfaces in ancient sequences. Barbados subsurface limestone samples, as a group, are characterized by a narrow range of δO18 compositions and a much wider range of δC13 compositions. This relationship may provide a means for identifying ancient limestones that have undergone subaerial diagenesis. Finally, our data indicate that positive δC13 PDB values are not inconsistent with diagenesis within the near-surface freshwater phreatic environment.

Relative elevation of late Pleistocene high sea level stands: Barbados uplift rates and their implications

Abstract Calculations confirm that the uplift of Barbados during the past 130,000 yr has been at nearly constant relative rates in the Clermont and Christ Church standard traverses, and that sea levels responsible for Barbados terraces I (82,000 yr B.P.) and II (105,000 yr B.P.) attained approximately the same level which was 20–25 m below the level represented by Barbados III (125,000 yr B.P.). Preference for the correlation of Barbados III with the prominent first interglacial 18 O peak in stage 5 is stated once again. Further, correlation with Eemian and Pangaion of the European pollen record is suggested.