Palaeotempestology: Beach ridges, corals and sclerosponges

After looking in depth at lake sediment layers as a proxy for hurricane activity, I’ll now turn my attention to the marine environment, as we head the seaside in our investigation. As we move off shore into the ocean, there are some other proxies as we broaden our options for looking at past tropical cyclones. For example, large scale storm surge or precipitation events can lead to rapid erosion or landslides which may become trapped in sediment records in the ocean. Corals can be smashed and broken in a storm and deposited or trapped in mud substrates.

Rubble Ridges

A ridge that is largely made up of broken coral or shell in mud layers is called a chenier. The subtle differences between a beach ridge and a chenier are described in the introduction of a paper by Taylor and Stone in 1996. Basically, it describes a chenier as having muddy swales in between ridges of sand, coral and shell deposits over the layers of sediment from the normal active geomorphological processes. Beach ridges however, are long ridges aligned with the general approach of the waves and confined by the limits of tidal depositing. 

Taylor and Stone (1996) describe how beach ridges and cheniers have formative processes during normal tidal and swell events, but ridge formation above the high tide level is likely to be due to extreme tropical cyclone action via extra deposition of sand, coral or shell. It is also likely that in the tropics, most or even all, cheniers are built by tropical cyclones (or the rare tsunami). It is this fact that allows them to add to the jigsaw of palaetempestological data, when appropriate examples are found.

Coral rubble ridges can also provide eveidence of storm history. If the bathymetry allows, we can see ridges of left behind by storms, which will likely contains larger proportions of coral debris. An excellent example of this is again found in Taylor and Stone 1996 where they have examined Curacao island in the Great Barrier Reef in Austalia (Figures 1 and 2). Figure 1 is a view to show the sheltered side of the island which suffers less wave action (and so mainatains a historical record) and therefore can accumulate beach and coral ridges during storm surge events. These ridges can be radiocarbon dated to provide the chronology in Figure 2 (labelled as Figure 3 in Figure 1).

Figure 1: Curacoa Island in the Great Barrier Reef, Australia showing coral rubble ridges. Source: Taylor and Stone 1996

Figure 2: Coral rubble ridges from transect denoted in Figure 1, showing radiocarbon dates of each mound.Source: Taylor and Stone 1996

These data can be misleading however, and although provide a clear demonstration of large surge events, in periods of high storm frequency, multiple storms will be superimposed on top of one another as it takes time for the sediments to become resistant to further storm erosion. This resistance is through carbonate cementation via weathering of coral material. This is an example of the one of the sources on uncertainty in using this data.

Frequency is studied by looking at the interval between ridges, but Nott and Hayne in 2003, also developed a proxy for intensity of storms. It links the height of the ridge with the minimum flood depth due to storm surge, which is above the highest tide level. The paper suggests their identified ‘super cyclones’ are much more frequent than previously considered along the Great Barrier Reef.

However, a key and more stable marine proxies in the near-coasts zones affected by tropical cyclone landfalls, is found through drilling cores both in corals and sclerosponges. A combination of coral core data and examination of beach and coastal zone sediments can be a powerful duo when assessing coastal impacts.

Correlating with corals

Normally, these cores are taken from old specimens in the areas most frequently affected by tropical cyclones. Many studies have been conducted on corals and sclerosponges in the Caribbean and across the North Atlantic coast of the U.S. prone to tropical cyclone activity, as well as pacific basins also prone to tropical cyclones. A list of datasets on coral and sclerosponges is compiled by NOAA’s National Climate Data Center and shows the range of study undertaken.

Layers of growth can be examined for stable isotopes or metal deposits which can tell us much about the past characteristics of the uppermost levels of the marine environment. One of the key pieces of information relevant to tropical cyclone formation that we can gain from coral cores, is the proxies for sea surface temperature (SST). SST is one of the main near-coast environmental components for generating storms that make landfall i.e. if the waters are warmer in the North Atlantic, perhaps during a La Nina phase of the El Nino-Southern Oscillation, then conditions are more favourable for cyclogenesis (birth of a cyclone) which gives a higher likelihood of landfall if occurring near to the coast. Information from corals is highly relevant and adds to the gamut of data that are used to build palaeotempestological records.

Diving for data

This has to be one of the more appealing ways to gather climate data: Dive in to the warm tropical waters, search around the ocean floor looking for suitable corals or sponges, retrieve your sample (Figure 3) followed by some lab analysis over a rum cocktail – sign me up! Of course, as with any worthwhile endeavour, there is only a very small amount of time spent in the field doing the fun stuff, compared to the lab work and analysis to follow.

Figure 3:  Coring large Porites coral, Rowley Shoals, Western Australia (Photo credit: Eric Matson, AIMS)

The equipment used (as shown in Figure 3) is custom built and after finding a suitable specimen, based on age, size, shape and species. It is often difficult to find multiple samples for verification purposes - a particularly tricky element of this type of study.

Due to limitations in the field, it is difficult to know whether a sample is of high or low quality, as it is not always obvious where interruptions in growth cycles, infestations, or damage from marine life are present. Samples are returned to the lab to have X-ray images taken and for chemical analysis. Two main factors are derived from corals. Firstly, there is the growth rate based on samples with clear banding, and secondly the information via the geochemical composition of  various layers of the coral skeleton which represent different times in its life cycle.

Figure 4 shows a slice of a Porites coral illuminated by ultraviolet light to show luminescent banding associated with freshwater inputs from heavy precipitation events which lead to local flooding and therefore more terrestrial-based organic material being made available.

Figure 4: Coral slice illuminated by UV light showing luminescent banding which indicated freshwater input after flood events. Source: Lough, 2010 John Wiley & Son s, Ltd

A good NOAA summary of how sclerosponges are used to reconstruct past climate can be found when the practice was still quite young in 1998 can be found here in the proceedings from a workshop held in Miami.

Another method used with sponges and corals is to analyse stable oxygen and carbon isotopes. Rather than luminescence, this examines the stable oxygen isotopes within their carbonate skeletons, which are formed according to their surroundings and 'locked-in' as a record. The ratio between Oxygen-18 (heavy and abundant in sea water) and Oxygen-16 (lighter and abundant in clouds and water vapour), can tell us a lot about sea surface temperatures. This ratio depends on temperatures since the fractionation between the isotopes occurs via evaporation and condensation. Since most evaporation occurs in the tropics, we see less and less oxygen-18 in the atmosphere as we head towards the poles as the heavier isotopes tends to condense out first. 

Using coral cores, one of the most important factors to consider is that the process by which corals and sponges incorporate oxygen into their skeletons (as Calcium Carbonate mainly) also prefers the heavier oxygen-18 isotope. This is corrected for using other biochemical characteristics. Once calibrated, the coral core can reveal information about past temperatures as corals  tend to preferentially utilise the heavier isotope in colder water. This therefore allows us to infer sea surface temperature from coral cores and link our data to past events such as strong La Nina conditions or fluctuations in the Pacific Decadal Oscillation (PDO). For more detail, this NASA Earth Observatory educational web page on Oxygen isotopes.

So after coral and sponge samples have been retrieved and analysed how do we actually gain some useful information?

How do corals tell us about tropical cyclones?

There are a number of studies that use coral luminescence. This sounds like a fairly abstract concept at first but the rationale is fairly straight forward so let me try to explain:

Luminescence can be a strong indicator of flooding in rivers near to the sample sites. It is this property of coral records that becomes very useful for palaeotempestology in that flood events that affects a large area, are likely to be caused by severe storms, monsoonal changes. This begins to tell us about the extremes of any given climatic conditions in the coral’s history. Climate variability is known to shift rainfall patterns, such as during ENSO phases or monsoon rains, and so this can lead to modulation of rainfall amounts over land, which correlate well with coral luminescence. These luminescent lines also act as ground for comparison for other coral records. 

Work by Johan Nyberg (2002) and Barnes et al. (2003) are examples of using coral luminescence to infer tropical cyclone activity in certain parts of the world. These papers explain the process of measuring luminescence and how the data can be applied to various fields of study surrounding past climates and climate variability.

The method of using UV light to identify terrestrial run off events in coral was first identified by Peter Isdale in 1984, Isdale identified that strong banding did not occur in corals greater that 20km from the coast and that the brighter bands correlated with periods of high precipitation and therefore enhanced riverine outflow to the sea.

Coral information combined with studies of tree rings can provide good cross validation and increase confidence in building past chronologies of climatic events such as monsoon droughts as studied by D’Arrigo et al. 2008, and so I wonder if there is anything that can help us find out about past wet seasons, or climate modes, such as the PDO, as in Rodriguez-Ramirez et al. 2014,or ENSO as in D’Arrigo et al. 2006, and therefore link to storm activity.

In my next blog, I will explore the use of tree rings to find out about past storms.