Long-term Sea Level Change Research Articles

Mitigating the effects of vertical land motion in tide gauge records using a state-of-the-art GPS velocity field

This study aims to correct for long-term vertical land motions at tide gauges (TG) by estimating high-accurate GPS vertical velocities at co-located stations (GPS@TG), useful for long-term sea-level change studies and satellite altimeter drift monitoring. Global Positioning System (GPS) data reanalyses are mandatory when aiming at the highest consistency of the estimated products for the whole data period. The University of La Rochelle Consortium (ULR) has carried out several GPS data reanalysis campaigns with an increasing tracking network, an improving processing strategy and the best methodology. The geodetic results from the latest GPS velocity field estimated at ULR (named ULR5) are presented here. The velocity field includes 326 globally distributed GPS stations, from which 200 are GPS@TG (30% more than previous studies). The new GPS data processing strategy, the terrestrial frame definition and the velocity estimation procedures are described. The quality of the estimated vertical velocities is empirically assessed through internal and external velocity comparisons, including the analysis of the time-correlated noise content of the position time series, to be better than 0.6mm/yr (2 sigma). The application of this velocity field is illustrated to appraise to what extent vertical land motions contaminate the estimates of satellite altimetry drifts. The impact on the altimeter-derived sea level trends was evaluated to be up to 0.6mm/yr. Worldwide TGs were grouped into regions in order to explore long-term spatial sea level variability in the rates of sea level change. By taking into account the vertical land motion of the tide gauges, the dispersion of the observed sea level rates within each region was reduced by 60%. Long-term regional mean sea level variations up to 70% from the global mean were found.

Evidence of nearby supernovae affecting life on Earth

Observations of open star clusters in the solar neighborhood are used to calculate local supernova (SN) rates for the past 510 million years (Myr). Peaks in the SN rates match passages of the Sun through periods of locally increased cluster formation which could be caused by spiral arms of the Galaxy. A statistical analysis indicates that the Solar System has experienced many large short-term increases in the flux of Galactic cosmic rays (GCR) from nearby supernovae. The hypothesis that a high GCR flux should coincide with cold conditions on the Earth is borne out by comparing the general geological record of climate over the past 510 million years with the fluctuating local SN rates. Surprisingly a simple combination of tectonics (long-term changes in sea level) and astrophysical activity (SN rates) largely accounts for the observed variations in marine biodiversity over the past 510 Myr. An inverse correspondence between SN rates and carbon dioxide (CO$_2$) levels is discussed in terms of a possible drawdown of CO$_2$ by enhanced bioproductivity in oceans that are better fertilized in cold conditions - a hypothesis that is not contradicted by data on the relative abundance of the heavy isotope of carbon, $^{13}$C.

Long-term sea level change in the Malaysian seas from multi-mission altimetry data

The long-term sea level change during 1993 to 2008 was investigated in the Malaysian seas from satellite altimetry data of the TOPEX, JASON-1, ERS-1, ERS-2 and ENVISAT missions. Sea level data retrieval and reduction were carried out using the radar altimeter database system (RADS). In RADS data processing, the 2008 updated environmental and geophysical corrections were applied. Six 1° × 1° areas were chosen for the altimetry data comparison and to find the best ocean tide model for the Malaysian seas, where the altimeter tracks are nearby tide gauge locations. Similarity in the patterns of sea level variations indicates good agreement between tide gauge and altimetry data when using the FES2004 ocean tide model. It also shows that altimetry data is perfectly suited to investigate long-term sea level change in the Malaysian seas. Here, sea level variations for four areas in the Malaysian seas have been investigated using 15 years of altimetry data. The altimeter sea level time series revealed that since 1993, the mean sea level in Malaysian seas has been rising at a geographically-dependent rate varying from 1.4 to 4.1 mm/yr. This kind of information is important for studying environmental issues related to flood investigations and global warming especially for an area that until now have not gotten much attention from the altimeter science community.

Correction to “Long‐term and recent changes in sea level in the Falkland Islands”

In the paper “Long-term and recent changes in sea level in the Falkland Islands” by P. L. Woodworth et al. (Journal of Geophysical Research, 115, C09025, doi:10.1029/2010JC006113, 2010), in paragraph 47 we adopted a value of −0.52 mm/yr for the estimated rate of present-day sea level change in the Falkland Islands due to glacial isostatic adjustment (GIA). This value was used to remove the contributions of GIA to our measurements of historical and recent rates of sea level change. However, it was based on a misreading of the data file of Peltier [2004] on the Permanent Service for Mean Sea Level Web site (http://www.psmsl.org/train_and_info/geo_signals/gia/peltier). More reasonable values to apply to the observed changes since the mid-nineteenth century and in recent years would be −0.69 and −0.61 mm/yr respectively. Consequently, the long-term rate of sea level change between 1842 and the early 1980s, after correction for air pressure effects and for GIA, reported as +0.75 ± 0.35 mm/yr in paragraphs 1, 47, 55, and 61 should be +0.92 ± 0.35 mm/yr, the corresponding rate between 1842 and the midpoint of recent data of 1.06 ± 0.22 mm/yr in paragraphs 48 and 55 should be 1.23 ± 0.22 mm/yr, and the corresponding rate since 1992 reported as 2.51 ± 0.58 mm/yr in paragraphs 1 and 52 becomes 2.60 ± 0.58 mm/yr. The middle of paragraph 63 becomes “The Stanley data suggest that the rate of change of sea level in East Falkland since 1992 has been approximately 2.6 mm/yr, a rate supported by information from satellite altimetry.” These small GIA model corrections have no bearing on the main findings of our paper on the difference in the rates of sea level change between the historical (1842 to present-day) and recent (last 2 decades) epochs

Evidence for a slow subsidence of the Tahiti Island from GPS, DORIS, and combined satellite altimetry and tide gauge sea level records

Monitoring vertical land motion is of crucial interest in observations of long-term sea level change and its reconstruction, but is among of the most, yet highly challenging, tasks of space geodesy. The aim of the paper is to compare the vertical velocity estimates of Tahiti Island obtained from five independent geophysical measurements, namely a decade of GPS and DORIS data, 17 years sea level difference (altimeter minus tide gauge (TG)) time series, ICE-5G (VM2 L90) Post-Glacial Rebound (PGR) model predictions, and coral reef stratigraphy. Except for the Glacial Isostatic Adjustment (GIA also known as PGR) model, all the techniques are in a good agreement and reveal a very slow subsidence of the Tahiti Island averaged at −0.5 mm/yr which is barely significant. Nevertheless, despite of that vertical motion, Tahiti remains an ideal location for the calibration of satellite altimeter measurements.

Land motion estimates from GPS at tide gauges: a geophysical evaluation

Space geodesy applications have mainly been limited to horizontal deformations due to a number of restrictions on the vertical component accuracy. Monitoring vertical land motion is nonetheless of crucial interest in observations of long-term sea level change or postglacial rebound measurements. Here, we present a global vertical velocity field obtained with more than 200 permanent GPS stations, most of them colocated with tide gauges (TGs). We used a state of the art, homogeneous processing strategy to ensure that the reference frame was stable throughout the observation period of almost 10 yr. We associate realistic uncertainties to our vertical rates, taking into account the time-correlation noise in the time-series. The results are compared with two independent geophysical vertical velocity fields: (1) vertical velocity estimates using long-term TG records and (2) postglacial model predictions from the ICE-5G (VM2) adjustment. The quantitative agreement of the GPS vertical velocities with the ‘internal estimates’ of vertical displacements using the TG record is very good, with a mean difference of ~0.13 ± 1.64 mm yr~1 on more than 100 sites. For 84 per cent of the GPS stations considered, the vertical velocity is confirmed by the TG estimate to within 2 mm yr~1. The overall agreement with the glacial isostatic adjustment (GIA) model is good, with discrepancy patterns related either to a local misfit of the model or to active tectonics. For 72 per cent of the sites considered, the predictions of the GIA model agree with the GPS results to within two standard deviations. Most of the GPS velocities showing discrepancies with respect to the predictions of the GIA model are, however, consistent with previously published space geodesy results. We, in turn, confirm the value of 1.8 ± 0.5 mm yr~1 for the 20th century average global sea level rise, and conclude that GPS is now a robust tool for vertical land motion monitoring which is accurate at least at 1 mm yr~1.

High-resolution records of the response of coastal wetland systems to long-term and short-term sea-level variability

While it is well known that coastal systems respond to long-term sea-level changes, the importance of short-term sea-level dynamics is often overlooked. Year-to-year variability in annual mean sea level along the North American Atlantic coast is part of a regionally consistent pattern that is coupled to low atmospheric pressure and high wind field anomalies persisting over 100s to 1000s of km. These short-term sea-level dynamics, along with long-term sea-level changes are shown to be closely coupled to a set of high resolution excess 210Pb geochronologies from four physiographically distinct salt marsh estuaries surrounding Long Island, NY, USA. However, the degree to which a marsh responds to either forcing depends on its physiographic setting. Accretion and mineral deposition rates in marshes situated in embayments with long fetches and low-tidal ranges are shown to respond most to the short-term dynamically driven changes in sea level. On the other hand, accretion and mineral deposition in a marsh in an embayment with a high-tidal range and reduced fetch best track the long-term changes in mean sea level, presumably because the physiography limited the meteorological drivers of short-term sea-level change. The close coupling between marsh accretion, physiographic setting and indices of sea-level change indicates that these coastal system respond both differently and rapidly (2–5 yr) to climate variability.

Carboniferous-Permian long-term sea-level change inferred from Panthalassan oceanic atoll stratigraphy

This paper traces late Palaeozoic second-order sea-level change based on the stratigraphy of the Akiyoshi Limestone, an accreted Carboniferous-Permian Panthalassan atoll carbonate succession in SW Japan. The estimated subsidence of a volcanic edifice and the variable rate of carbonate accumulation reveal the long-term sea-level history of the late Palaeozoic. During Bashkirian time, just after the lowstand stage at the mid-Carboniferous boundary, a slow progressive long-term sea-level rise began. This sea-level rise then increased greatly during Moscovian time. The following Kasimovian to Asselian interval represents a stable highstand stage. Beginning in the Sakmarian, sea level fell slowly and became stable in the Yakhtashian (= Artinskian). This stable sea level was maintained into the Midian (= Capitanian), although a small-scale sea-level rise of approximately 55–70 m is recognized in the Murgabian (= Wordian). The most noteworthy aspect of this change of sea level is the rapid sea-level rise during the Moscovian that created a very large accommodation space and resulted in accumulation of a thick carbonate succession. The eustatic rise from the earliest Bashkirian lowstand to the latest Moscovian highstand had an amplitude of approximately 230–240 m. Such a large-scale eustatic rise in the long-term sea-level change would most likely be caused by greater uplift of the ocean floor along the mid-oceanic ridges, as a result of an increase in the production of oceanic crust during that time.

Influence of dynamic topography on sea level and its rate of change

Mantle fl ow likely supports up to 2 km of long-wavelength topographic relief over Earth’s surface. Although the average of this dynamic support must be zero, a net defl ection of the ocean basins can change their volume and induce sea-level change. By calculating dynamic topography using a global mantle fl ow model, we fithat continents preferentially conceal depressed topography associated with mantle downwelling, leading to net seafl oor uplift and ~90 ± 20 m of positive sea-level offset. Upwelling mantle fl ow is currently amplifying positive dynamic topography and causing up to 1.0 m/Ma of sea-level rise, depending on mantle viscosity. Continental motions across dynamic topography gradients also affect sea level, but uncertainty over the plate motion reference frame permits sea-level rise or fall by ±0.3 m/Ma, depending on net lithosphere rotation. During a complete Wilson cycle, sea level should fall during supercontinent stability and rise during periods of dispersal as mantle fl ow pushes continents down dynamic topography gradients toward areas of mantle downwelling. We estimate that a maximum of ~1 m/Ma of sea-level rise may have occurred during the most recent continental dispersal. Because this rate is comparable in magnitude to other primary sea-level change mechanisms, dynamic offset of sea level by mantle fl ow should be considered a potentially signifi cant contributor to long-term sea-level change.

Facies stacking patterns in high-frequency sequences influenced by long-term sea-level change on a Permian Panthalassan oceanic atoll: An example from the Akiyoshi Limestone, SW Japan

Detailed core examination of Middle Permian (Guadalupian) lagoonal successions of mid-Panthalassan atoll carbonates (Akiyoshi Limestone) revealed distinct stratigraphic variation of facies and sequence stacking patterns. The long-term sea-level change reconstructed from the atoll stratigraphy of the Akiyoshi includes a Bolorian to Midian gentle sea-level fall, punctuated by a short-lived Murgabian sea-level rise. These sea levels can be regarded as eustasy, judging from the monotonous subsidence of the Akiyoshi atoll and its good correspondence with the sea-level curves of other areas. Two sequence types are defined. Sequence type A consisting of shoal to lagoon grainstone occurred in low accommodation during the sea-level highstand-to-falling stages of the late Murgabian to Midian. This sequence type is bounded by two successive subaerial exposure surfaces and exhibits the uneven but generally low δ 13C values throughout the section. In contrast, sequence type B occurred in increasing accommodation during the sea-level rise of the early Murgabian. This sequence type represents a composite sequence comprising parasequences and consists of various types of facies such as peritidal lime-mudstone, lagoonal packstone and boundstone, and shoal grainstone. Sequence type B is characterized by consistently high δ 13C values except for the negative shift immediately below the sequence-top surface. These lines of evidence demonstrated that long-term eustatic sea-level change obviously controls the variations of sequence types in atoll carbonates.

Physiographic Control on the Development of Spartina Marshes

The established paradigm indicates that low-lying marsh production is primarily regulated by submergence and the resulting edaphic conditions. The physical sedimentary environment is in turn deemed to affect production through its effect on marsh-surface elevation relative to sea level. However, our results working with Spartina anglica indicate that production and marsh perennation are controlled by variation in surface level relative to the plant's meristematic base and not hindered by prolonged submergence. These findings suggest that plant responses to changes in sedimentary conditions determine marsh development and survival in the short term, independently of long-term changes in relative sea level.

Salt Marsh Dieback: An overview of recent events in the US

This paper provides an overview of the marsh dieback events that have been observed along the east and gulf coasts of the U.S. over the past decade. It is likely that some of the recently reported changes in marsh vegetation were affected by physical or biotic disturbances that are known to generate bare areas, such as overgrazing or wrack smothering. Other areas may be experiencing a state change such as that caused by long-term changes in sea level. However, sites in many areas are not readily explained by these causes and are considered to have experienced “sudden dieback.” In such cases, there are observations that the above-ground plant material thinned or browned or, in some cases, failed to re-emerge in the spring; the dieback occurred over a period of months and usually affected multiple sites within the area; and there is evidence that these events are transient (through successful transplants or natural regrowth/recovery), although some areas take years to recover. We explored the potential linkage of dieback with drought (as characterized by the Palmer Severity Drought Index), and found that there is evidence for an association in the southeast (GA and SC) and the Gulf (LA), but not in the mid-Atlantic (DE, VA) or northeast (ME, RI, CT). We also review the evidence for potential causes of sudden dieback, including changes in soil chemistry, fungal pathogens, top–down consumer controls, and multiple stressors. There is currently no single explanation that can be applied to recent dieback. We highlight the need for the development of improved diagnostics that will allow us to better classify dieback areas and provide evidence for (or against) potential causes.

Drilling to Decipher Long-Term Sea-Level Changes and Effects—A Joint Consortium for Ocean Leadership, ICDP, IODP, DOSECC, and Chevron Workshop

No abstract available. doi:10.2204/iodp.sd.6.02.2008

Drilling to Decipher Long-Term Sea-Level Changes and Effects – A Joint Consortium for Ocean Leadership, ICDP, IODP, DOSECC, and Chevron Workshop

No abstract available. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.6.02.2008" target="_blank">10.2204/iodp.sd.6.02.2008</a>

Global scale patterns of continental fragmentation: Wilson's cycles as a constraint for long-term sea-level changes

We propose to characterize land–ocean distributions over Late Proterozoic to Phanerozoic times from measurement of perimeters and areas of continental fragments, based on paleomagnetic reconstructions. These measurements serve to calculate geophysically constrained breakup and scatter indexes of continental land masses from 0 to 1100 Ma. We then provide quantitative investigation and modelling of relationships between scatter of continental landmasses and mean age of the oceanic lithosphere during Mesozoic times, which appears to range from 56 to 62 Ma over the last 170 My. We then inverse the scatter of continental landmasses in terms of global oceanic crust mean age over the last 600 My, i.e. back in times where no measurement of seafloor accretion history is possible because of subduction. We finally show that the inferred evolution of oceanic lithosphere mean age over the Phanerozoic remarkably correlates in time with long-term sea-level changes since the Cambrian.

Some important issues to do with long-term sea level change

This paper gives an overview of several important issues concerned with the measurement and understanding of long-term sea level change, which is one of the most important topics in sea level science. The issues discussed span science, technology and our ability to fund and operate the national and global observation networks required for a proper understanding of long-term change and its impacts. Many of these topics were explored at a 'Celebration of UK sea level science' at the Royal Society on 16-17 February 2004, and are discussed further in a number of papers in this volume.

From Cycles to Sequences: Sequence Stratigraphy and Relative Sea Level Change for the Late Cambrian of the North China Platform

Abstract In the Late Cambrian, the North China Platform was a typical carbonate ramp platform. The Upper Cambrian of the northern part of the North China Platform is famous for the development of bioherm limestones and storm calcirudites and can be divided from bottom to top into the Gushan, Changshan and Fengshan formations. In this set of strata, the deep-ramp mudstone and marls and the shallow-ramp packstones and grainstones constitute many carbonate meter-scale cycles of subtidal type. More tidal-flat dolomites are developed in the Upper Cambrian of the southern margin of the North China platform, in which limestone and dolomite beds also constitute many carbonate meter-scale cycles of the peritidal type. These cycles are marked by a variety of litho-facies successions. There are regularly vertical stacking patterns of meter-scale cycles in long-term third-order sequences, which is the key to discerning such sequences. Third-order sequence is marked by a particular sedimentary-facies succession that is the result of the environment-changing process of deepening and shoaling, which is genetically related to third-order sea level changes. Furthermore, four third-order sequences can be grouped in the Upper Cambrian of the North China Platform. The main features of these four third-order sequences in the northern part of the platform can be summarized as follows: firstly, sequence-boundaries are characterized by drowning unconformities; secondly, the sedimentary-facies succession is generally constituted by one from deep-ramp facies to shallow-ramp facies; thirdly, a succession of “CS (?)+HST” (i.e., “condensed section and high-stand system”) forms these four third-order sequences. The chief features for the third-order sequences in the southern part of the North China Platform comprises: more dolomites are developed in the HSTs of third-order sequences and also developed more carbonate meter-scale cycles of peritidal types; the sedimentary-facies succession of the third-order sequences is marked by “shallow ramp-tidal flat”; the sequence boundaries are characterized by exposure punctuated surfaces. According to the changes for the third-order sequences from the north to the south, a regular sequence-strati graphic framework can be established. From cycles to sequences, the study of sequence stratigraphy from litho-facies successions to sedimentary-facies successions exposes that as follows: meter-scale cycles that are used as the basic working unit actually are litho-facies successions formed by the mechanism of a punctuated aggradational cycle, and third-order sequences that are constituted by regularly vertical stacking patterns of meter-scale cycles are marked by sedimentary-facies successions. On the basis of the changing curve of water depth at each section, the curve of the relative third-order sea level changes in the late Cambrian of the North China Platform can be integrated qualitatively from changing curve of water depth. The correlation of Late Cambrian long-term sea level changes between North China and North America demonstrates that there are not only similarities but also differences, reflecting control of long-term sea level changes both by global eustacy and by regional factors.

Satellite remote sensing of earthquake, volcano, flood, landslide and coastal inundation hazards

Satellite remote sensing is providing a systematic, synoptic framework for advancing scientific knowledge of the Earth as a complex system of geophysical phenomena that, directly and through interacting processes, often lead to natural hazards. Improved and integrated measurements along with numerical modeling are enabling a greater understanding of where and when a particular hazard event is most likely to occur and result in significant socioeconomic impact. Geospatial information products derived from this research increasingly are addressing the operational requirements of decision support systems used by policy makers, emergency managers and responders from international and federal to regional, state and local jurisdictions. This forms the basis for comprehensive risk assessments and better-informed mitigation planning, disaster assessment and response prioritization. Space-based geodetic measurements of the solid Earth with the Global Positioning System, for example, combined with ground-based seismological measurements, are yielding the principal data for modeling lithospheric processes and for accurately estimating the distribution of potentially damaging strong ground motions which is critical for earthquake engineering applications. Moreover, integrated with interferometric synthetic aperture radar, these measurements provide spatially continuous observations of deformation with sub-centimeter accuracy. Seismic and in situ monitoring, geodetic measurements, high-resolution digital elevation models (e.g. from InSAR, Lidar and digital photogrammetry) and imaging spectroscopy (e.g. using ASTER, MODIS and Hyperion) are contributing significantly to volcanic hazard risk assessment, with the potential to aid land use planning in developing countries where the impact of volcanic hazards to populations and lifelines is continually increasing. Remotely sensed data play an integral role in reconstructing the recent history of the land surface and in predicting hazards due to flood and landslide events. Satellite data are addressing diverse observational requirements that are imposed by the need for surface, subsurface and hydrologic characterization, including the delineation of flood and landslide zones for risk assessments. Short- and long-term sea-level change and the impact of ocean-atmosphere processes on the coastal land environment, through flooding, erosion and storm surge for example, define further requirements for hazard monitoring and mitigation planning. The continued development and application of a broad spectrum of satellite remote sensing systems and attendant data management infrastructure will contribute needed baseline and time series data, as part of an integrated global observation strategy that includes airborne and in situ measurements of the solid Earth. Multi-hazard modeling capabilities, in turn, will result in more accurate forecasting and visualizations for improving the decision support tools and systems used by the international disaster management community.

Facies architecture and high-resolution sequence stratigraphy of an Upper Cretaceous platform margin succession, southern central Pyrenees, Spain

Excellent exposures of Upper Cretaceous (Santonian) carbonate platforms on the northern flank of Sant Corneli anticline (southern central Pyrenees) provide detailed information of facies architecture in both depositional strike and dip directions. Basic accretional units are differentiated by facies contrast across mappable surfaces. These surfaces do not show clear evidence of subaerial erosion and are correlated basinward with bedding planes across which there are subtle changes in skeletal composition. Two types of basic accretional units have been identified based on bedding patterns, internal facies architecture and skeletal composition: (1) Rudist buildups consist of a rudist and coral belt at the platform margin, passing landward into a slender-hippuritid lithosome, locally overlain by a bioclastic blanket that passes basinward, into bioclastic “apron-like” clinobeds and into fine-grained packstone/wackestone. (2) Calcarenite wedges consist of yellow-brown, benthic-foraminifer-rich grainstones to grain-dominated packstones, with scattered rudist shells and small coral colonies, passing basinward into mud-dominated packstones to wackestones, with variable siliciclastic content (quartz sand to silt and clay). Rudist buildups and calcarenite wedges alternate, although not rhythmically. These changes in platform skeletal composition reflect changes in the dominant type of carbonate-producing biota independently of the changes in accommodation. Both types of basic accretional units: rudist buildups and calcarenite wedges, form simple sequences and parasequences according to internal lithofacies arrangement and inferred sea-level cyclicity (cycles or paracycles). High-frequency sea-level cyclicity fits in the Milankovitch frequency band. Long-term changes in accommodation governing aggradation, progradation and backstepping of basic sequences and parasequences reflect tectonic influence rather than long-term changes in eustatic sea level.

Sea level, sediments and the composition of seawater

The composition of seawater changed significantly during the course of the Phanerozoic in concert with long-term changes in sea level, which were largely the result of changes in the volume of mid-ocean ridges. These reflect changes in the rate of seafloor spreading and determine the rate of seawater cycling through mid-ocean ridges. Spencer and Hardie (1990) and Hardie (1996) proposed that the changes in the cycling rate of seawater through mid-ocean ridges have been responsible for the changes in the composition of seawater during the Phanerozoic. This explanation cannot account for the major changes in the composition of seawater during the Cenozoic, which was a time of nearly constant seafloor spreading rate. However, changes in the rate of deposition of shallow water carbonates and their penecontemporaneous dolomitization can explain the Cenozoic changes in seawater composition. This indicates that sedimentologic parameters have played a major role in the chemical evolution of seawater. The more rapid cycling of seawater through MORs during the Mesozoic can explain semiquantitatively the fairly minor differences between the composition of seawater in the Eocene and in the Jurassic. Changes in the composition of seawater during the Paleozoic were probably determined both by changes in the rate of dolomite and gypsum/anhydrite deposition and by changes in the rate of seawater cycling through mid-ocean ridges. If the causes of these compositional changes are the same as those during the Cenozoic and Mesozoic, then sedimentologic factors have been significantly more important than the rate of cycling of seawater through MORs in shaping the chemical evolution of seawater during the Phanerozoic.