Orson van de Plassche

Timing and magnitude of recent accelerated sea-level rise (North Carolina, United States)

We provide records of relative sea level since A.D. 1500 from two salt marshes in North Carolina to complement existing tide-gauge records and to determine when recent rates of accelerated sea-level rise commenced. Reconstructions were developed using foraminifera-based transfer functions and composite chronologies, which were validated against regional twentieth century tide-gauge records. The measured rate of relative sea-level rise in North Carolina during the twentieth century was 3.0–3.3 mm/a, consisting of a background rate of ~1 mm/a, plus an abrupt increase of 2.2 mm/a, which began between A.D. 1879 and 1915. This acceleration is broadly synchronous with other studies from the Atlantic coast. The magnitude of the acceleration at both sites is larger than at sites farther north along the U.S. and Canadian Atlantic coast and may be indicative of a latitudinal trend.

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.

Optical dating of dune sand for the study of sea-level change

A new, widely applicable method to obtain well-constrained relative sea-level records, presented here, relies on optical dating of inorganic coastal landforms and sediments. Eighteen optical ages of basal dune sand from a barrier in Massachusetts, United States, indicate that local relative sea level rose 8 m during the past 5.5 k.y. A sea-level curve drawn from these optical ages shows good agreement with independent evidence from salt-marsh peat sampled behind the barrier, demonstrating the reliability of the method. Optical dating will be particularly useful in analyzing coastal records that contain too few organic indicators to allow high-resolution, 14 C-based analyses.

The use of Jadammina macrescens (Brady) and Balticammina pseudomacrescens Bronnimann, Lutze and Whittaker (Protozoa: Foraminiferida) as sea-level indicators.

In studies of foraminiferal distributions in saltmarshes in New England and Atlantic Canada, Balticammina pseudomacrescens and Jadammina macrescens have formerly been grouped as Trochammina macrescens. This study recognises them as distinct species and shows that they have significantly different distributions on the surface of saltmarshes in Maine. Here, relative abundances of J. macrescens correlate with tidal elevation (r2=0.55, p=0.00), whereas those of B. pseudomacrescens do not (r2=0.03, p=0.34). Therefore, J. macrescens is a better sea-level indicator in Maine marshes than B. pseudomacrescens. In Menunketesuck River marsh, Connecticut, a clear vertical zonation of saltmarsh foraminifera is absent, which hinders interpretation of fossil data. This study shows that, in Maine, separating B. pseudomacrescens and J. macrescens in counts of fossil data facilitates the detection of transgressional and regressional events in the biostratigraphical record and, hence, the reconstruction of local sea-level histories. Regional variability between northern and southern New England saltmarshes necessitates the establishment of local reference sets of modern analogue data to aid palaeoecological interpretations.

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.

Evolution of the intra-coastal tidal range in the Rhine-Meuse delta and Flevo Lagoon, 5700-3000 yrs cal B.C.

Abstract The evolution of the coastal and intra-coastal tidal range in the western Netherlands during the period 5700-3000 yrs cal B.C. (6875-4400 yrs B.P.) is considered by comparing one coastal mean high water curve and two local mean high water curves (MHW) with respect to three alternative relative mean sea level (MSL) curves: (1) the existing 1982-MSL curve for the western and northern Netherlands, (2) a MSL curve based on the oldest part of the 1982-MSL curve and the youngest part of a recently published MSL curve based on data from the east-central coastal plain, and (3) a hypothetical MSL curve for the east-central coastal area assuming crustal subsidence of 0.10 m/1000 yr relative to the 1982-MSL curve. With regard to coastal tidal range, the first and third MSL curve yield a similar result: a minor increase in coastal tidal range between 5200 yrs cal B.C. (6200 yrs B.P.) and 4200 yrs cal B.C. (5350 yrs B.P.). The second MSL curve gives an increase in coastal tidal range from an assumed value of 2 m by 5200 yrs cal B.C. (6200 yrs B.P.) to ca. 3 m by 4100 yrs cal B.C. (5300 yrs B.P.), conflicting with existing model results. Intra-coastal tidal damping was a widespread phenomenon and played an important role in the evolution of the coastal plain. For the first and second MSL curves, tidal attenuation in the Rhine-Meuse delta and the east-central coastal sector begins around 5250 yrs cal B.C. (6275 yrs B.P.); with the third MSL curve, the reduction of the tidal range starts is 500 years later. The first and third MSL curves result in complete tidal damping in the Rhine-Meuse delta and the east-central coastal area; with the second MSL curve, the tidal range in the Rhine-Meuse delta remains ca. 1.3 m, whereas in the east-central area tidal reduction is complete. In spite of these different results, it is clear that the period 5300-4300 yrs cal B.C. (ca. 6375-5400 yrs B.P.) was very important for the subsequent evolution of the West Netherlands coastal plain given that (a) the relative MSL rise slowed down from about 0.8 m/100 yr to 0.23 m/100 yr, (b) the coastal tidal range increased, possibly by as much as 1 m, resulting in higher sediment input into the intra-coastal area, while (c) further landward transportation of sediment was hampered due to intra-coastal tidal damping, which reduced rates of local MHW rise, tidal current velocities and accommodation space.

Influence of Relative Sea-Level Change and Tidal-Inlet Development on Barrier-Spit Stratigraphy, Sandy Neck, Massachusetts

ABSTRACT Sandy Neck is a 10-km-long sheltered mesotidal barrier spit along the southeastern New England coast, bordering Cape Cod Bay. Its stratigraphy contains evidence of relative sea-level (RSL) rise and tidal-inlet development, and enables reconstruction of accretion and progradation of this barrier through time. Core, trench, and ground-penetrating-radar (GPR) data show that the lower boundary of the uppermost, eolian, facies climbs about 9 m from the proximal to the distal part of the spit, away from the mainland anchor point. This change is thought to be related primarily to a comparable rise in RSL. The same boundary also shows a shallowing trend in a seaward direction, indicating net spit progradation through time. The absence of inlet-channel facies in the stratigraphy of the proxima part of the barrier spit, in contrast to the presence of inlet facies in more distal parts of the spit, indicates that some time elapsed after barrier initiation before an inlet thalweg developed. In its earliest stages, this thalweg was poorly defined, wide, and shallow, but its characteristics changed as the barrier spit lengthened. In the central part of Sandy Neck, the assemblage of inlet-channel facies becomes markedly thicker, reflecting thalweg deepening in response to paleotopographic and substrate-lithologic factors. The stratigraphy of the distal part of the spit shows the effects of continued, but more gradual, inlet deepening that may be tied directly to increasing barrier length. The process-controlled stratigraphy presented here differs from current facies models because i accounts for temporal changes in coastal environmental processes and parameters. It suggests a high preservation potential for proximal eolian facies in the spit sequence, underneath a cover of fine-grained and organic back-barrier sediment. Once reliably dated, the new stratigraphic data will help to improve the regional RSL curve for the southern Cape Cod Bay area.

Water-level changes in the Hammock River marsh palaeovalley, Connecticut between 13,000 and 6000 (14C) yrs B.P.

Abstract Detailed analysis of basal organic deposits underlying Hammock River marsh, Connecticut allowed documentation of water-level changes that occurred between 13,000 and 6000 yrs B.P. Four main periods of groundwater- and lake-level movements and related environmental changes can be identified. 1. (1) 12,500-10,200 yrs B.P. (lake stage): very rapid rise of the groundwater table of about 2 to 3 m, resulting in a shallow lake, followed by a more gradual rise of about 2.5 to 1.5 m. 2. (2) 10,200-7000 yrs B.P. (freshwater marsh, stage 1): slow overall rise of the water table interrupted by a drop of at least 1 m between about 9500 and 9000 yrs B.P. and of at least 0.8 m between about 8000 and 7500 yrs B.P., each event leading to oxidation and maceration of organic material. 3. (3) 7000-6400 yrs B.P. (complete desiccation of the swamp): rapid fall of the water table of at least 3.9 m. causing large-scale down-wasting of the accumulated peat. 4. (4) After 6400 yrs B.P. (freshwater marsh, stage 2): rapid rise of the water table. The water-table rise of period 1 and the lowering of period 3 are attributed to predominantly local causes, while the groundwater fluctuations during period 2 are probably climate-related. The final water-level increase reflects the influence of Holocene relative sea-level rise.