8. Sea level fluctuations

Evidence of past positions of sea level

Being close to the acme of a Pleistocene interglacial period, we are today experiencing high sea levels which have been exceeded on only a few occasions during the past 1 million years or so. Now is, therefore, not the most opportune  time to examine field evidence of past sea level positions. Other than in regions of the globe where the land has been uplifted, the majority of such  evidence is submerged and buried by modern  marine sediments.

The best indications that sea levels at Bermuda have been lower  than at present are provided by features which formed above sea level but are now submerged, such as fossil soils and cave formations including stalagmites and stalactites. Evidence of the very lowest sea levels are now deeply submerged and, therefore rarely observed. Limited investigations using small research submarines have revealed wave-cut features at 60m and 115m below present sea level suggesting the existence of ancient shorelines at these depths (IL1).

Evidence of past high sea levels from sedimentary deposits

That sea levels at Bermuda were higher than at present on several occasions is evident  from ancient marine deposits, for example beaches, which today occur above the elevation of equivalent modern deposits. Beach features, such as the beach step (Figures 4g and 4h, Chapter 4) can provide a relatively  precise record of sea level position at the time of its formation.  Erosional features (Figure 8a) and accumulations of debris, formed by wave action, can also be useful, if less precise, “markers”, of past high sea levels.  More information on ancient marine deposits and their role in establishing past high sea level positions on Bermuda is provided in Chapter 4.

Figure 8a. A wave-cut erosional feature preserved in an ancient beach near Cloverdale, South Shore, Smith’s Parish. The near vertical surface immediately to the right of the green pen is a cliff, or scarp, cut into an upper beach/foredune of Belmont Formation age by storm waves. The sloping strata to the left of the pen represents the sand which  avalanched in a seaward direction as the scarp became unstable. This is known as a talus slope. In combination these features provide reliable evidence that approximately 200,000 years ago storm waves removed a large portion of a beach and that, for a period, average sea level stood close to present sea level, allowing for a storm surge of 2 to 3 metres. (Scale: green pen provides scale)

Biological evidence

Robust biological evidence of past high sea level events is puzzlingly rare on Bermuda, given the existence of other forms of  evidence, discussed above.  In many other parts of the world, “raised” or “emergent” fossil coral reefs are “go to” indicators of past high (relative) sea levels. Yet Bermuda is virtually devoid of examples of ancient coral reefs preserved above present sea level. Those that have been reported have not been well documented and cannot be verified.

Albeit scarce, biological sea-level imprints do exist. Examples are found at Watch Hill Park in Smith’s Parish  and on the shore of the Great Sound near Fort Scaur in Sandys Parish. At these localities, fossil  molluscs which in life  clung to, or bored into, inter-tidal or sub-tidal rock surfaces are today preserved intact, and articulated, “high and dry” above present sea level (Figure 8b).

Figure 8b.  Biological evidence of  a past high sea level at Watch Hill Park, Smiths Parish. At this locality, just over 6 m (20 ft) above present sea level, fossil marine molluscs and encrusting vermetid worms (not shown here) are preserved in the position in which they lived, clinging to an inter-tidal rock surface at a time when sea level was significantly higher than that of today. The molluscs are lithophagic bivalves (Brachidontes domingensis), approximately 2 cm (1 in) in length.

Evidence from caves

Some of the most reliable indicators of past sea level positions – lower and higher than today – are found in Bermuda’s caves. Cave formations, or speleothems, such as stalagmites and stalactites  form in air-filled chambers by precipitation of calcite from dripping or trickling water. A submerged speleothem is, therefore, proof of a past lower sea level.

Interruptions in speleothem growth can be related to climatic events but  flooding of the cave by ground water in response to high sea levels is another likely cause.  Careful analysis of the precipitated calcite layers within speleothems and age determination by U-series radiometric dating  can reveal the time-span of interruptions in their growth. Geological investigations  on Bermuda (HA1,WA1) have identified speleothem growth-interruptions, which have been attributed to submersion and, thus, have added significantly to our understanding of the sea level history.

Figure 8c. Cross section through a Bermuda stalagmite.  This 0.3m (1ft) high stalagmite collected from Squeeze Cave was sliced through in order to expose and date (by the U-series method) the internal layers. Where there is a large time gap between consecutive layers a period of submersion, i.e. a rise in sea level, may be indicated. Changes in colour between the layers is related to variations in the chemistry of the infiltrating rain water.

Geochemical evidence

Small limestone islands such as Bermuda are saturated with ground water (fresh or saline) up to sea level. Above sea level the limestone is unsaturated. These two conditions – when the pores of the limestone are filled with water and when they are drained – are respectively termed phreatic and vadose. The dividing line, or plane, between them is known as the water table which lies close to sea level (within 0.5 m). Near the coast, sea water saturates the limestone below the water table and is responsible for marine phreatic cementation, which deposits high-Mg (Magnesium) calcite cement in the rock pores. This produces a vertically constrained  geochemical signature unique to coastal limestones which have been submerged below the water table. This signature has been used as a marker of past sea level positions on Bermuda, most notably the high sea level at 200,000 years ago responsible for deposition of the Belmont Formation marine deposits (LA2,ME2).

The history of sea level fluctuations

Global climate has fluctuated dramatically since the beginning of the Pleistocene Epoch 2.5 million years ago . Approximately every 100,000 years vast ice sheets formed and advanced across the continents of the Northern hemisphere.

During cold “glacial” periods large volumes of water were bound up in the ice sheets which covered up to one third of the earth’s land area. This water was released as meltwater during warm “inter-glacial” periods. In response, global sea levels rose and fell over a range of up to 120 m (390 ft) (Figure 8c). Rates of sea-level change sometimes exceeded 100 mm (4 inches) per decade.

At each glacial period Bermuda would have emerged as a single large island, we can call “Greater Bermuda” (Figure  1f, Chapter 1). The North Lagoon which is presently submerged to depths of 15 m to 20 m (60 ft) would have been transformed into a forested plateau. Neighbouring Argus, Challenger and Bowditch “banks” would have temporarily emerged from the sea.

Fossil soil layers and peat deposits buried within the sediments of the present Platform, or North Lagoon, (VO2,GI3) attest to the occasions (at each “Ice Age”) when the land area of Bermuda expanded to more than ten times its present size, as the lagoon waters drained away. Submerged remains of cedar trees found in St George’s Town Cut, Mills Creek and at Gurnet Rock represent more direct evidence of episodic lower sea levels that saw the transformation of the present sea bed into a forested landscape.

When sea levels were high, during warm interglacial periods, the land area of Bermuda would have been close to its present size. Elevated ancient beach deposits and erosional features, mentioned earlier,  indicate that on several occasions in the past Bermuda’s shoreline closely corresponded to that of today. And for relatively brief periods of time, sea levels at Bermuda exceeded its present level (Figure 8c). Where suitable unaltered coral fragments have been found in elevated marine conglomerates, these have been the source of robust ages attributed to high sea level events as described in Chapter 4.

Nonetheless, the data have been challenged and the history of high sea level events at Bermuda has been the subject of intense debate. The best approximation to a consensus on Bermuda’s record of (relative) high sea level positions is as follows: +21 m (+ 70 ft) at 400,000  bp (years before present) (HE3); >+6 m (>+20 ft) at 200,000 bp (ME2,RO4); +5 to +6 m (~ 18 ft) at 120,000 (HA1,HE3,MU4) ; 0 to +1 m (~ +2 ft) at 80,000 bp (MU4,VA2). This information is summarised by the dotted line in Figure 8b. It should be noted that the data from Bermuda indicate positions – generally considered to be peak positions – of sea level at particular times in the past. There is evidence of other positions of sea level, for example during the penultimate Interglacial period, approximately 200,00 years ago,  which record other short duration, “high frequency”, oscillations . 

Sea level curve
Figure 8c. The mid- to late-Pleistocene eustatic global  sea level curve (modified from Siddall et al (SI1)).  Superimposed on this chart is a plot of high relative sea level positions that have been documented on Bermuda, represented by the dotted line. It can be concluded that relative sea level at some of the major interglacial periods over the last 500,000 years were anomalously high at Bermuda. Glacio-isostatic adjustment (GIA) – an effect which causes the oceanic crust around Bermuda to subside during interglacial periods – has been advanced as a mechanism which might contribute to  such anaomolies.

Eustacy versus Isostacy

Rising and falling sea-levels during the Pleistocene Epoch were associated respectively with the onset of warm Interglacial periods and cold Glacial periods. Sea level change resulted on the one hand from the release of water to the oceans from melting continental ice sheets and, on the other hand, from the transfer of ocean water (as snow) to the land when ice sheets were re-forming.  Respective increases and decreases in the volume of water in the world’s oceans caused “eustatic”  sea level changes simultaneously across the globe.

Vertical movement of a land mass causes changes in “relative” sea level. This is the level of the sea as measured against a datum on the land. During the Pleistocene, the cause of such changes  at Bermuda were attributable, at least in part, to the loading and unloading of the North American continent by massive ice sheets which grew up to 3 km (2 miles) thick. Such changes in relative sea level in response to land movement are termed “isostatic”. The effect on Bermuda is explained in more detail below.

During glacial periods the great weight of glacial ice on the continental crust in the northern hemisphere caused it to downwarp and by so doing it squeezed the underlying viscous mantle material laterally outward creating a “forebulge”. Upon melting of the ice sheets, during inter-glacial periods, the continental crust rose up, or rebounded, as it was relieved of the weight of ice. Regions that had previously been covered with glaciers therefore experienced a fall in relative sea level. Presently, at the end of a period of deglacatiation in places like Canada, Scandinavia and Scotland, relative sea level is therefore  falling. At most other localities around the world sea level is rising. These are the differences between regions where land movement, or isostacy, is dominant and regions where sea level movement, or eustacy, is dominant.

There are other less direct isostatic influences on relative sea level imposed by the Pleistocene climatic cycles.  Bermuda, for example, while not having been subjected directly to ice loading, is close enough to North America to have been affected by the weight of large ice-sheets which episodically accumulated there. Viscoelastic mantle material was squeezed towards Bermuda from beneath North American crust, creating a forebulge, during glacial periods and, subsequently flowed back during inter-glacial periods (Figure 8d). The resultant cycle of isostatic uplift and subsidence of Bermuda exaggerated the eustatic effect on sea level. 

The glacio-isostatic effect at Bermuda was on a scale of tens of metres at the most, compared to the eustatic effect which was on a scale of over one hundred metres. Nevertheless, the isostatic effect at Bermuda, which was largely unrecognised or dismissed in 20th Century research, is significant. Glacio-isostatic adjustment (“GIA”) is now considered (DU1, WA1) to have contributed to a relative sea level history at Bermuda which is unique (Figure 8c). 

Figure 8d. The isostatic response of the Bermuda Seamount to glaciation and deglaciation on the North American continent.   Viscoelastic mantle material below the North American plate is displaced away from North America under the load of ice during glacial periods, and Bermuda is uplifted.  The reverse occurs during interglacial periods when the load of ice is removed from North America.