Patterns of Sea Level Change

The effects of sea-level change are generally seen across the continental shelf and upper slope. They are exposed to subaerial processes as sea level falls. The ensuing marine invasion renews the sedimentary regime that was abandoned after the previous high stand.

From this point in the course, we will be using the concept of geological time. Geology deals in term of a span of time longer than 4.5 billion years. An excellent source for understanding geological chronology is the web time machine

Mesozoic and Tertiary Sea Level Change

Seismic-stratigraphic studies at Exxon led to proposal of a global sea-level history for Triassic through present time (originally in Vail and Mitchum and later revised by Haq , et al. 1988 From the Triassic to the present, they identified more than 100 global sea-level changes.

While agreement on the magnitude or timing of the older sea level changes is far from universal, the various generations of the Vail and Haq sea-level curves provide a useful point of reference. These curves group sea-level variations into cycles that are reflected in the seismic records of the world's shelf margins. First-order cycles coincide with times of major continental plate break-up and seem to last 200 to 300 million years. More frequent and irregular transgressive and regressive events are superimposed on these longer cycles.

According to Haq, et. al., 1988 the last time the world ocean reached its present level was in the lower Triassic (ca. 250 million yBP). Midway through this period (lower Cretaceous), sea level reached a highstand roughly 100 meters below today's ocean. At about this time, the new and relatively shallow Atlantic Ocean was opening, and gradually replacing a deep Pacific Ocean. Using spreading-rate data for the mid-Atlantic over the past 180 m.y., Hays and Pitman 1973 proposed that the Cretaceous high sea-level stand (ca. 125 MyBP) and the subsequent lowstand (95 MyBP) could be explained solely by expansion and contraction of the mid oceanic ridge system. It should be kept in mind, however, this was also a time of rapid warming in what has been described as a "greenhouse climate" similar to that of today. The relative importance of glacio-eustatic versus tectonic controls remains a topic of debate.

Since the mid-Cretaceous lowstand, sea level oscillations have continued with a rough periodicity of 100,000 years superimposed on a gradual rise . The Oligocene highstand was accompanied by a roughly continuous equatorial seaway that spawned a cosmopolitan coral fauna in the Caribbean region that has been in decline ever since. The curve of Haq, et al. 1988 hints at an increase in the frequency of sea-level oscillations since the mid-Cretaceous, perhaps reflecting an increase in the relative importance of astronomic factors.

Pleistocene Sea Levels

The Pleistocene glacial and interglacial events were first described from Alpine deposits and a chronology of glaciations was established in which four glacial events were named for the southern extent of continental glaciers: Penck & Bruckner, 1909 in Europe, the Gunz, Mindel, Riss and Wurm intervals; in North America, the Nebraskan, Kansan, Illinoian and Wisconsinian glaciations. The Wisconsin and Illinoian glacial stages are still accepted and widely used for glacial stratigraphy and correlation in North America and elsewhere. However, the Kansan and Nebraskan are not longer considered valid glacial stages in light of much evidence for more complicated pre-Illinoian glaciation of the central United States. Glacial ice may have began forming as early as the Oligocene but certainly by Pliocene time. The sea level curves of glacial formation and melting show a downward trend in volume of seawater available as more water became locked in continental glaciers. The concept of four glacial events has been revised as studies of deep-sea sediments disclosed a long succession of glaciations during the Pleistocene; the most recent of which was shown by radiocarbon dating to equate with deposits of the last glaciation on the continents. Studies by Phleger, et al. as early as 1953, demonstrated that long Atlantic cores of deep-sea sediments recorded at least nine Pleistocene ice ages. This led to a long series of investigations of Pleistocene sea level history. Paleomagnetic dating of deep-sea cores shows that about 30 glacial events occurred during the Pleistocene.

Three different approaches were used in the investigation of Pleistocene glaciations:

The studies by different investigators were merged into a large project, CLIMAP, which has given us a reasonable reconstruction of the Pleistocene history, and causes of Pleistocene glaciations. The history of this project was described in Imbrie and Imbrie which is the basic reference for the following discussion. The concept of astronomical forcing was reaffirmed by Broecker and Denton 1990 who linked the formation of North Atlantic Deep Water to oscillations in the temperature record and by Crowley and Kim. 1994

Ericson, et al. 1964 used the abundance of selected indicator species that they considered to be sensitive to changes in climate to analyze the Pleistocene and to determine the climatic history of the Pleistocene. Geochemical analyses and the absence of an indicator species, Globorotalia menardii, established the 11,000 yBP boundary that we now call the Holocene. They also established what they considered the boundary of the onset of the Pleistocene, marked by the extinction of discoasters, at about 1.5 MyBP. However, in the same cores, the differences from the results by Emiliani 1966 were significant. Emiliani used the technology developed by Epstein, et al., 1953 for measuring oxygen isotope ratios (O18:O16) in foraminifera, which was assumed to be an indicator of paleotemperatures. Ericson's and Emiliani's methods agreed over the more recent part of the Pleistocene record, but over the older parts, the results did not agree.

Broecker and van Donk 1970 conducted an isotopic analysis of the same core which allowed the results of Emiliani, Ericson and Imbrie to be compared. Some environmental factor other than surface water temperature caused Globorotalia menardii to appear and disappear cyclically in deep water of the Atlantic Ocean, so Ericson was not measuring water temperature. The results led to the conclusion that much of the isotopic variation noted by Emiliani must be due to changes in the volume of the ice sheets -- not to changes in temperature. When an ice age begins, the continental ice sheets grow, removing water and the ocean water left behind becomes enriched in oxygen18. The calcium carbonate shells of marine organisms reflect the water's isotopic composition at the time that they were formed. The higher the ratio of O16 to O18 in shells, the more land ice was present when the sediment was laid down.

Examination of uplifted coral reef terraces instead of cores during the same period of time was yielding data about the last 200,000 years of Pleistocene history. Broecker showed three dates for high sea levels --the present starting 5,000 to 7,000 yBP, 80,000 yBP and 120,000 years ago from reef terraces in Florida and Hawaii. Matthews 1968, 1988 examined terraces in Barbados and determined that they were accretionary and that each represented reef growth at a former sea level. Two terraces were dated at 80,000 and 125,000 years, with a third between the others dated at 105,000 yBP. The 105,000-year peak did not fit the Milankovitch 65o insolation curve. Broecker, et al., 1968, 1970 then computed a 45o N insolation curve in which precession is given more weight than tilt. The warm peak at 50 KyBP (from the 65o N curve) was removed and a new peak appeared at 106 KyBP. The results clearly indicated that the last four high sea stands (122, 103, 82, and 5 KyBP), correspond closely in time to the last four prominent warm peaks (127, 106, 82, and 11 KyBP) in the modified curve of summer insolation, not only in chronology but also in magnitude. Ku, et al. 1990 confirmed the Pleistocene high stands between 80,000 and 125,000 yBP. This matching with the coral terraces of Barbados, New Guinea, Australia and Hawaii was one of the convincing arguments in support of the Milankovitch theory. Marine terraces in Haiti, Puerto Rico, and Jamaica, have elevations and ages that agree with the deep-sea record. Peteet et al. 1992 used coral data to study the initiation of the last glacial episode and concluded that orbital forcing alone was not adequate to initiate the ice sheet growth seen in their model reef terrace sea level.

The major cycle in the isotopic record was 100,000 years and this primary climatic cycle has an asymmetrical, saw-toothed shape in which glacial expansions were terminated by rapid warming. This pattern suggests that climate responded to some continual forcing oscillation, with the most obvious being variation in the amount of sun energy received by the earth coupled with positioning of the continents.

Several investigators presented data to establish the initiation of the Pleistocene. The Pleistocene-Pliocene boundary is defined in deep-sea sediments by:

However, dating was still unresolved until magneto-stratigraphy and ice fluctuation measurement data were used together. Two points on the magnetic time scale were important; the Brunhes-Matuyama boundary, dated at 700,000 years ago and the Olduvai normal event dated at 1.8 MyBP. The working group of Opdyke, Foster, Glass and Hays found cores that would be long enough to penetrate the Brunhes-Matuyama boundary and in which Hays had determined radiolaria zones. The magnetic signal was present in these cores and the reversal boundaries fit Hays's radiolarian zones. The first appearance of cold-water species in southern Italy (where G. truncatulinoides first appear in abundance) coincides with the Olduvai Normal Event, and the beginning of the Pleistocene was established at 1.8 MyBP.

Holocene Sea Level Rise

The Holocene includes the last 11,000 years, since the end of the last major glaciation. There is some debate over the low point of the previous Wisconsinian sea level. Some early studies off the New England coast placed the lowstand near 130 meters. Other investigations that take into account the local effects of glacial loading beneath the glacier and compensatory uplift further away place that low closer to 85 meters (Dillon & Odale, 1968 in New England; Moerner, 1971 in Scandinavia).

The ensuing sea-level curve is generally characterized by a gradual rise in sea level, although the exact character and the continuity of that rise continue to be debated. One group of researchers proposes a rapid sea-level rise (8-10 mm/yr = 8-10 m/thousand years) from 12,000 to about 6,000 yBP, followed by a decrease in the rate of rise to 1-1.5 mm/yr between 6,000 yBP to 3,000 yBP and slower yet thereafter.

Another group suggests that the Holocene sea level was more ragged and either stopped, slowed dramatically or even dropped to form marine terraces at levels 40, 20, 15 and 9 meters below present sea level. Logically, the rates of rise during the time intervals between successive stillstands had to be much faster than those calculated for a continuous rise. Within the group favoring an irregular sea-level curve, two investigators proposed a fairly recent highstand of the world ocean two meters above its present level. Following the idea of episodic change, Blanchon proposed that the rate of Holocene sea level rise could be hindcast from changes in coral type in reef cores and the thickness of branching- and massive-coral sections found in the cores. While this is an intriguing idea, the authors have seen all the proposed sequences occurring within a single area over the same time frame, with the patterns being controlled by the paleoenvironment in which the reef developed (i.e., fore reef versus backreef).

All of these curves had little data on sea level before about 10,000 yBP. Fairbanks 1989 filled this gap with a curve based on core data from Acropora palmata in deeply submerged reef terraces off Barbados. The curve contains significant gaps in the depositional record from terrace to terrace which he interpreted to reflect sudden rises in sea level due to the rapid addition of meltwater into the world's oceans. Central to Fairbanks' interpretation is the idea that the terraces associated with the Barbados reefs were actually formed by the development of Acropora palmata reefs on a smooth slope as rising sea level suddenly stopped; this was followed by a sudden rise which left the reefs behind. An alternative possibility is that the reefs formed on older terraces and that each reef represents the accretion of coral as sea level passed by a pre-existing notch in the otherwise smooth island slope. In this second scenario, the gaps in the record would be the result of reefs not being able to accrete along steep intervening slopes. One alternative supports a smooth sea level rise, while the other points to pulses in the addition of meltwater to the world oceans. While the picture is far from a clear and universally accepted, there seems to be wider acceptance of the constant rate of sea-level rise, which was first described in an Atlantic curve by A. Conrad Neumann. 1971 Neither concept supports a highstand of sea level during the past 3,000 - 6,000 years. In contrast, while Chappell 1983 favored a smooth sea level rise for the Holocene of eastern Australia, he proposes that by 6,000 yBP sea level rose to a point 2 meters above present and has gradually dropped to its present position in the intervening years (in agreement with Ters 1973 and Fairbridge; 1962 but a smooth, continuous decline).

The important point is that sea level has generally risen over the past 18,000 years from a lowstand somewhere 100 m below today's oceans. This has had a dramatic effect on the rates and styles of deposition in both the deep sea and adjacent ocean margins over time. The debate on the specific patterns and controls of Holocene, Pleistocene and older sea-level changes is still very active. While everyone generally agrees on the types of factors that can potentially impact sea level at any particular time, how these interact to produce a single depositional sequence remains at the heart of scientific discussions. Our models are further complicated by the fact that localized changes in even one of these factors can produce relative sea level curves that are markedly different from one site to the next.

Global Warming and Present Sea Level Change

Regional studies of modern sea level are generally consistent in their overall picture, but not in the details. Much of the disagreement is related to the local factors described above. While scientists might debate the specifics, they are in universal agreement about one thing - both global temperature and sea level continue to rise today, the latter at a rate of approximately 1-3 mm/year on a global scale similar to that for the late Holocene .

Even this modest rate of rise has had a significant effect on coastal and nearshore areas when private homes and public facilities have been built close to the water's edge. In Cape Hatteras National Seashore, the protection and repair of private homes were perennially subsidized by public funds until the Federal government realized that keeping ahead of the retreating shoreline in the face of rising sea level was a futile effort. In other areas where enormous investments have been made in private development or public infrastructure (i.e., Miami Beach), federal funds are still being expended to either fortify the shoreline with concrete walls or to replace beaches washed away by seasonal storms and the slow rise of the global tide. Eventually, these will be abandoned as well, once the cost of stemming coastal loss exceeds the revenues generated by those facilities.

More problematic is the question of how much of this rise is part of a larger, natural cycle and how much is the result of man-induced changes in the planet. Temperature records over the past century show a warming trend . Furthermore, a strong correlation has been demonstrated between this pattern and anthropogenic activities such as the burning of fossil fuels, increasing nitrous oxide and methane emissions from factories and automobiles, the use of chlorofluorocarbon-based aerosol propellants and other activities related to the progressive industrialization in our modern society.

These are highly debated points. Three graphs are presented, but not discussed. Look at these and see what you think.

The September 4, 1990 Time Magazine presents new data and a new view of global warming together with figures from NOAA research. Since 1980 temperatures have climbed in the Arctic faster than the rest of the earth, and climatic changes are expected to move more rapidly in the polar region. Warming in the Arctic is a factor in the global climate system because the difference in temperatures between the tropics and the poles drives the system. Formation of the North Atlantic deep water gives off staggering amounts of heat in the North Atlantic and sends water into abyssal depths. At 14,000 yBP the pattern of ocean and atmosphere changed; oceanic circulation shifted dramatically and glaciers in both hemispheres began retreating, starting global warming. Studies have shown that this current was shut down until the last ice age ended.

The Younger Dryas, 11,000 yBP, links the transport of fresh water and ocean circulation. In as little as 100 years, northern Europe reverted to glacial conditions. To cause this event, the NADW system had stopped, and the warm intermediate depth water that supplies Europe's extra heat no longer flowed. A massive influx of fresh water from the melting North American glaciers was diverted to the St. Lawrence seaway when the normal Mississippi route was blocked (this blocked sinking of the water mass and formation of NADW). This was recorded in the change from low O18 /O16 ratio in Gulf of Mexico foraminifera to a high ratio (the meltwater discharging from the Mississippi was 016 rich). The event shows linking of freshwater flow, ocean circulation, and climate, but the effect was only regional.

If the poles continue to warm faster than the tropics, the circulatory system may diminish as the pull created by the sinking NADW decreases. Concurrent with this the movement of warm water in the Gulf Stream could slow or stall reducing temperatures in North America and Europe. The melting 11,000 years ago sent freshwater from the St. Lawrence River into the North Atlantic to create the 1,300-year Younger Dryas. It has been suggested that this new Arctic melting could follow the same pattern. A significant result of such a change is that during the Younger Dryas the monsoon weakened in Asia and the Sahara expanded - a change in precipitation accompanied the climate event and this could have dramatic effects. This type of result is presented as an alternate view to the end result of modern global warming.

Large sums of money have been invested in research to model the possible changes in global climate over the coming decades. The magnitude of the proposed changes in these "Global Climate Models" (GCM'S) varies considerably depending on the assumptions that are made and the particular model that is used. Regardless of the details, however, the existing record shows a marked increase in the rate of change over recent decades. And whichever model is used, a less than optimistic prognosis is associated with a scenario in which "business as usual" continues. As a best guess, global temperature would rise by 3 C over the next century with a worst-case estimate of 5 C. If the worst-case scenario proves true, then the world ocean could rise by a meter in the next century, at a rate tenfold that of the past 3,000 years. The impact on the world's weather patterns would have staggering impacts on business, and especially agriculture. Low-lying areas would be flooded at a rate too high for even the US Army Corps of Engineers to keep up with. Coral reefs might be left behind, exposing now-protected port areas to increasing wave energy. Combined with recent estimates that hurricane wind speeds could increase by 7-15 mph with a 2.2 C increase in global temperature, this portends for dramatic coastal-engineering problems. Because many important fisheries are related to reefs, implications to that industry are profound.

The final paragraph from the Time article is important to all of us:

At the entrance to the Churchill Northern Studies Centre, a base for investigations of regional climate change, a rusting rocket is a mute reminder of the complex's earlier life as part of defenses against Soviet nuclear attack. That threat never materialized, and now, belatedly, scientists venture from the base to study a threat that has materialized but against which no adequate defense has been mounted. Despite the danger that climate change poses, the resources currently devoted to studying this problem - and combatting it - are inconsequential compared with the trillions spent during the cold war. Twenty years from now, we may wonder how we could have miscalculated which threat represented the greater peril.