Arie F. M. de Jong

Deciphering Holocene sea-level history on the U.S. Gulf Coast: A high-resolution record from the Mississippi Delta

Published Holocene relative sea-level (RSL) curves for the U.S. Gulf Coast are in mutual conflict, with some characterized by a smooth RSL rise akin to widely accepted eustatic sea-level curves versus others, including several recent ones, that are characterized by a conspicuous “stair-step” pattern with prolonged (millennium-scale) RSL stillstands alternating with rapid (meter-scale) rises. In addition, recent work in Texas and Alabama has revitalized the notion of a middle Holocene RSL highstand, estimated at 2 m above present mean sea level. An extensive sampling program in the Mississippi Delta (Louisiana) focused on the collection of basal peats that accumulated during the initial transgression of the pre-existing, consolidated Pleistocene basement. We used stable carbon isotope ratios to demonstrate that many of these samples accumulated in environments affected by frequent saltwater intrusion in the <30 cm zone between mean spring high water and mean sea level, and we selected plant macrofossils that were subjected to AMS 14C dating. Nearly 30 sea-level index points from a ∼20 km2 study area on the eastern margin of the delta suggest that RSL rise followed a relatively smooth trend for the time interval 8000–3000 cal yr B.P., thus questioning the occurrence of major RSL stillstands alternating with abrupt rises. Given the narrow error envelope defined by our data set, any sea-level fluctuations, if present, would have amplitudes of <1 m. Although a true middle Holocene highstand never occurred in the Mississippi Delta, the high level of detail of our time series enables a rigorous test of this hypothesis. Correction of our data set for a hypothetical tectonic subsidence rate of 1.1 mm yr−1 (assuming a constant subsidence rate compared to the tectonically relatively stable adjacent coast of Texas) leads to sea levels near 2 m above present during the time interval 6000–4000 cal yr B.P. However, this model also implies a RSL position near −2 m around 8000 cal yr B.P., which is inconsistent both with data of this age from Texas, as well as with widely accepted sea-level data from elsewhere. We therefore conclude that a middle Holocene highstand for the U.S. Gulf Coast is highly unlikely, and that the entire area is still responding glacio-isostatically, by means of forebulge collapse, to the melting of the Laurentide Ice Sheet.

A Revised Chronology for Mississippi River Subdeltas

Radiocarbon measurements by accelerator mass spectrometry relating to three of the four late Holocene Mississippi River subdeltas yielded consistent results and were found to differ by up to 2000 carbon-14 years from previously inferred ages. These geological dataare in agreement with archaeological carbon-14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework, which has implications for prehistoric human settlement patterns, coastal evolution and wetland loss, and sequence-stratigraphic interpretations.

Sea level–climate correlation during the past 1400 yr

We present a new mean-high-water curve for Hammock River marsh, Clinton, Connecticut, obtained by improving the age model for an existing record of relative marsh elevation based on foraminiferal analysis of a 1.8-m-long peat core. Unlike the earlier curve, the new curve confirms trend changes in mean-high-water rise during the past 1400 yr as noted for salt marshes 15 km farther west, suggesting a regional cause. These trend changes and century-scale mean-high-water variations in the Clinton record correlate positively with large-scale regional variations in sea-surface and summer-air temperature, indicating a link between sea level and the climate-ocean system. On the basis of the Clinton mean-high-water curve, we conclude that real sea level oscillated centimeters to decimeters on a century time scale over the past 1400 yr, was 25 ± 25 cm higher ca. a.d. 1050 (Medieval Warm Period) than ca. a.d. 1650 (Little Ice Age), and rose at a mean rate of ∼1 mmṁyr−1 over the past 350 yr; there was little or no rise during the cool 1800s.

How stable is the Mississippi Delta

Large deltas are commonly believed to exhibit rapid rates of tectonic subsidence, largely due to sediment loading of the lithosphere. As a result, deltaic plains are prone to accelerated relative sea-level rise, coastal erosion, and wetland loss. Hurricane Katrina’s devastation testifies to the severe threat that these processes pose to the Mississippi Delta, but the relative role of tectonics versus other mechanisms causing land subsidence remains elusive. Relative sea-level records derived from basal peat have the potential to quantify differential crustal movements over Holocene time scales with exceptionally high accuracy and precision. Here we present new sea-level index points from two study areas in the southwestern Mississippi Delta that essentially coincide with a recently published detailed relative sea-level record from the eastern part of the delta. Our results show that differential vertical movements among the three study areas have been only 0.1 mm yr 1 .W e compare our evidence with a recent sea-level compilation from the Caribbean, to a large extent based on data from areas that are tectonically stable. Our sea-level index points nearly coincide with the Caribbean data, showing surprising tectonic stability for considerable sections of the Mississippi Delta. However, the well-documented high subsidence rates in and near the birdfoot of the Mississippi Delta indicate that different conditions prevail there. The rapid wetland loss in coastal Louisiana is likely due, to a considerable extent, to the compaction of Holocene strata.

Salt-marsh erosion associated with hurricane landfall in southern New England in the fifteenth and seventeenth centuries.

Lithostratigraphic and radiocarbon data from the inland section of Pattagansett River Marsh, Connecticut, show that this sheltered part of the salt marsh underwent significant erosion twice during the past 600 yr, each time followed by rapid and complete infilling of the eroded space with tidal mud and low marsh and high marsh peat. We argue that the erosion cannot be attributed to increases in tidal prism or to lateral migration of tidal channels. The ±2σ age range (A.D. 1390–1470) for the first low marsh growth in the older regressive sequence agrees well with the age range (A.D. 1400–1440) for a hurricane deposit 60 km to the east. The younger regressive sequence is dated with the greatest probability to the period A.D. 1640–1670, i.e., shortly after the hurricanes of A.D. 1635 and 1638. Our conclusion that the most likely cause of the erosion was hurricane activity is relevant to paleostorm research and the study of marsh sensitivity to and recovery from storm erosion.

Radiocarbon Dating of Lime Fractions and Organic Material from Buildings

We have dated carbonate fractions and organic material from different types of mortar from two sites in Belgium. We demonstrate the difficulties in obtaining good dates from carbonate samples. We also discuss the need for new types of dating material when the mortar comes from contaminated and disturbed sites, where even charcoal can yield aberrant results.

Deposition of sapropel S1 sediments in oxic pelagic and anoxic brine environments in the eastern Mediterranean: differences in diagenesis and preservation

Sediments from a boxcore in the previously anoxic brine-filled Poseidon Basin, eastern Mediterranean, have been studied and compared to sediments deposited in a ‘normal’ eastern Mediterranean environment. The boxcore can be divided into three main sedimentary intervals based on AMS-radiocarbon ages, foraminiferal and geochemical zonations. From the base of the core upwards these are: (1) 12.3‐31.2 cm, organic-rich sediments redeposited from within the brine; (2) 6.6‐12.3 cm, sediment containing a ‘cold’ foraminifera fauna redeposited from above the brine into the basin while the brine was still present; (3) 0‐6.6 cm, oxic pelagic sediment accumulated since the reoxygenation of Poseidon Basin which occurred1800 yrs BP. Near the base of the latter unit, a Mn-oxide peak has formed and it marks the present boundary between oxic and suboxic environments. A progressive downward oxidation front, which is usually found in ‘normal’ sapropel S1 sediments, has never formed in Poseidon Basin sediments. This has resulted in the preservation of the relationship between organic carbon and organic-related trace elements, e.g. Se, in the organic-rich sediments of Poseidon Basin, whereas such a relationship has been obliterated in ‘normal’ sapropel S1 sediments. On the basis of the carbonate content as well as the Sr=Ca ratio, preservation of carbonates appears to be better in the brine sapropel sediments of BC15 than it is in ‘normal’ sapropel S1 sediments. The high opal content of BC15 shows that biogenic opal is also much better preserved. The overall lower Corg=Ba ratio in BC15 suggests a better preservation of barite relative to that of organic carbon in shallow brine sediments, but is as yet inconclusive for the organic carbon preservation potential of brine relative to ‘normal’ unoxidised sediments.

Reconstructing “background” Rates of sea‐level rise as a tool for forecasting coastal wetland loss, Mississippi Delta

The Mississippi Delta is one of the most vulnerable coastal regions in the world, with rapidly deteriorating wetlands and an increasing threat for the city of New Orleans due to accelerated relative sea-level (RSL) rise. Rational coastal forecasting and policy-making for this area requires a detailed understanding of the temporal and spatial dimensions of RSL change. However, considerable controversy currently exists about the nature of Holocene RSL rise along the Gulf Coast. We have collected new, high-resolution RSL data from the eastern part of the Mississippi Delta that show that this area experienced smooth and continuous RSL rise that gradually decreased during the Holocene, consistent with numerous observations worldwide. Here, we demonstrate the potential of such data for quantifying natural “background” rates of RSL rise that should be taken into account in coastal forecasting of such threatened environments.

Radiocarbon Dates from the Jewish Catacombs of Rome

This paper reports on the first chronological assessment of the Jewish Catacombs of the ancient Rome per- formed by accelerator mass spectrometry (AMS) dating of small-size charcoal fragments scattered in the mortar used for seal- ing off the graves in the Villa Torlonia Catacomb complex. The significance of the obtained 14C readings has been carefully evaluated by taking into consideration the known technologies of quicklime production during Roman and recent times. The new data are of great concern for providing evidence that the Jewish catacombs were used for burial since the first century AD, thus some two centuries prior to the period traditionally believed to be the starting point of burial in the Jewish catacombs of ancient Rome. Such a significant aging of the Jewish catacombs could result in a deep re-examination of the current under- standing of the beginning and the evolution of the custom of catacomb burial in both Jewish and early Christian communities in Rome.

Comparison of AMS 14C ages of organic deposits and macrofossils: a progress report

Abstract A comparative study of AMS 14 C ages of organic deposits (minerotrophic peat and gyttja) and macrofossils was performed in order to evaluate the magnitude of a number of sources of error that may be present in bulk sediment samples. Coexisting macrofossils were found to yield consistent ages. No significant age difference between macrofossils and bulk samples was observed, indicating that disturbing effects (root contamination, hard-water effects and input of reworked, older material) are of minor importance in the data set presented here.