Pteropods from the Caribbean Sea : dissolution as an indicator of past ocean acidification

Introduction Conclusions References


Introduction
The faunal responses to ocean acidification are still largely unknown, although experimental evidence reveals that a reduction in pH typically leads to a decrease in calcification rates of a number of, but not all organisms (Feely et al., 2004;Orr et al., 2005;Guinotte and Fabry, 2008;Turley et al., 2010).To date, little information is available about important planktic producers of calcium carbonate.Several studies have investigated coccolithophore and planktic foraminiferal responses, but only three species of the aragonite-producing thecosome pteropods have been considered (Fabry et al., 2008;Comeau et al., 2009Comeau et al., , 2010a, b), b).Here we demonstrate a relationship between the dissolution of pteropod shells and the oxygen isotope record through the last 250 000 yr.A diverse and abundant assemblage of pteropods and heteropods is Figures recorded from marine cores collected from the Caribbean Sea offshore Montserrat.A number of these cores contain intervals of well-preserved pteropods which are associated with the glacial periods of the Late Pleistocene (Messenger et al., 2010).These well-preserved levels appear to be of widespread significance and a response to global climate change.
The group of holoplanktic molluscs known as the Pteropoda consists of two orders; the shell-less gymnosomes and the shell-bearing thecosomes (Fig. 1).These two orders are now considered to be less closely related than originally thought, but the term "pteropod" is still widely used (van der Spoel, 1976;B é and Gilmer, 1977;Lalli and Gilmer, 1989).Thecosome pteropods are a common component of the water column throughout the world's oceans and can reach densities of up to 10 000 individuals per cubic metre (The Royal Society, 2005;Fabry et al., 2008).They are consequently important prey to a number of large cetaceans and commercial fish (The Royal Society, 2005).This study focuses on the species Limacina inflata, a common euthecosome (suborder of thecosomata) pteropod.
All euthecosome pteropods produce calcareous shells from aragonite, a polymorph of calcium carbonate, which is particularly susceptible to dissolution (50 % more susceptible than calcite): see Mucci (1983), Millero (1996), Morse and Arvidson (2002) and Kl öcker et al. (2006).This makes their shells extremely vulnerable to dissolution due to ocean acidification and also means that, upon death, their shells frequently dissolve as they settle through the water column to the sea floor.This limits their occurrence in sediments to water depths of less than 3000 m and also restricts their presence in the geological record (Curry, 1971;Herman, 1971;Berner, 1977).The distribution of the modern fauna is well known (B é and Gilmer, 1977) and "pteropod oozes" have been recognised for over one hundred years (Murray and Renard, 1891).
With current increasing levels of atmospheric CO 2 and the resulting ocean acidification (Orr et al., 2005;The Royal Society, 2005), pteropods with their aragonitic shells are the subject of renewed interest, since they are likely to be the most vulnerable of the major planktic producers of CaCO 3 .They are also likely to be the first planktic fauna Figures to experience persistent decreased CaCO 3 saturation states.As an important part of the food web, especially in the Arctic and Southern Oceans, their potential demise is of great significance.

Marine sediment cores from Montserrat
In March 2002, as part of a multi-disciplinary project on the volcanic activity on the island of Montserrat, the RV L'Atalante recovered a series of piston-cores from the ocean floor surrounding the island (Fig. 2).Of the 12 cores collected on the "Caraval Cruise", CAR-MON 2 provides the longest time record (Le Friant et al., 2008) (Fig. 3) gives an accurate record of the Marine Isotope Stages (MIS) back ∼250 000 yr BP and the record compares well with other studies (Imbrie et al., 1984;Prell et al., 1986).This δ 18 O profile has been verified using a limited number of AMS radiocarbon dates and 39 Ar/ 40 Ar radiometric dates (Le Friant et al., 2008).Using the >150 µm size fraction, counts of the planktic foraminifera have allowed the determination of the Globorotalia menardii zonation (Ericson and Wollin, 1956;Reid et al., 1996;Le Friant et al., 2008).Dissolution of Limacina inflata shells has been quantified throughout CAR-MON 2 using the scale published by Gerhardt and Henrich (2001).As the Limacina Dissolution Index (LDX) has only been used by a limited number of workers (e.g.Kl öcker et al., 2006) on "fossil" material, its calculation is now described.
Pre-processed and dried sediment (Le Friant et al., 2008) was used to collect just over 300 (or as many as were present) pteropod specimens from two size fractions (>500µm and 150-500 µm) at varying intervals.Only whole specimens that retained their protoconch and protoconch fragments were counted.Determination of the dissolution of the pteropod shells was made using the Limacina Dissolution Index (LDX) Figures

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Full which was devised by Gerhardt et al. (2000) and published as a scale by Gerhardt and Henrich (2001).This method involves the qualitative analysis of the surface of the pteropod shell on a scale of 0 to 5; 0 being a pristine, transparent, lustrous shell with a smooth surface and 5 being an opaque, white and completely lustreless shell with additional damage.At least 10 shells (max 30 shells) of adult Limacina inflata of a size of 300 µm or larger were allocated a value from this scale by the use of light microscopy for each sample.The average for each sample was then calculated to provide the LDX value.
3 Correlation of pteropod dissolution record CAR-MON 2 records two levels of particularly well preserved, abundant and diverse pteropods (Fig. 3), which have been documented previously (Le Friant et al., 2008;Messenger et al., 2010) but not studied in detail.The upper concentration of pteropods is found in MIS 2 and has been dated at around 25 000 yr BP (55-80 cm), with a peak in pteropod preservation at MIS 2.2 (∼20 000 yr BP).The lower concentration of pteropods is within MIS 6 (295-425 cm) with a peak in preservation at MIS 6.4, dated at about 150 000 yr BP The upper concentration of pteropods corresponds almost exactly with the "pteropod sands" reported by Chen (1968) from the Gulf of Mexico, Venezuela Basin and other occurrences in the Caribbean Sea, Mediterranean Sea and Red Sea.Chen (1968) suggests that their widespread occurrence was controlled by Late Pleistocene climate changes, although this work is often overlooked in the modern literature.
This latest Pleistocene occurrence of abundant pteropods has also been recorded in the Andaman Sea (Sijinkumar et al., 2010), in the Red Sea (Almogi-Labin et al., 1991), off-shore Florida (Gardulski et al., 1990), on the western flank of the Great Bahama Bank (Eberli et al., 1997;Messenger et al., 2010), on the Brazilian Slope (Gerhardt et al., 2000), in the Caribbean Sea (Haddad and Droxler, 1996), off-shore Somalia (Kl öcker and Henrich, 2006;Kl öcker et al., 2006) and in the South China Sea (Wang et al., 1997).In the cores from the South China Sea and the Caribbean Sea, the concentrations at ∼20 000 yr BP and 150 000 yr BP are both recorded, clearly 6905 Introduction

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Tables Figures

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Full demonstrating that this enhanced preservation of aragonitic fossils is of global significance and not the result of local variations in water chemistry (Peterson and Cofer-Shabica, 1987;Peterson, 1990, Broecker andClark, 2002;Sepulcre et al., 2009).Elsewhere in the CAR-MON 2 core, increased shell dissolution occurs during interglacial periods and is particularly poor during extreme stages, such as at MIS 5.5.In the Gulf of Aden (Core KL15), Almogi-Labin et al. (2000) record the near absence of pteropods during interglacials (MIS 13, 11, 9, 7, 5 and 1).The record from this core shows that pteropod maxima appear to be at the glacial/interglacial transitions (especially the MIS 6 to MIS 5 transition).Such deglaciation "spikes" have also been noted by Frenzel (1975) and Berger (1977Berger ( , 1990)).Berger (1977) describes this world-wide phenomenon as a pteropod-rich layer present at the end of the last glacial, although, the exact timing and cause of this event are in some dispute.Serre-Bachet and Guiot (1987) also linked pteropod preservation to colder periods.This link is particularly striking in the post-MIS 2 records in the N.E.Atlantic Ocean (Ganssen and Lutze, 1982;Ganssen et al., 1991), Equatorial Atlantic Ocean (Kassens and Sarntheim, 1989) and the N. W. Indian Ocean (Kl öcker et al., 2006).This preservation relationship to colder periods is, almost certainly, due to fluctuations in pCO 2 causing higher pH during glaciations (Sanyal et al., 1995;Ruddiman, 2001;H önisch and Hemming, 2005;Yu et al., 2007) and lower pH during interglacials.In the CAR-MON 2 data there are some unexpected excursions from the general trend, which show that variations in dissolution are not directly proportional to the δ 18 O signal.This can be seen particularly between MIS 5.1 and 5.5, where changes in dissolution appear to be accentuated.Several factors during the sedimentation process, which are summarised in Fig. 4, may have influenced the LDX dissolution profile.The pattern produced by the LDX profile could not, however, be an artefact of sea floor dissolution and diagenesis.If pteropods within CAR-MON 2 showed a general trend from LDX 0-2 in the nearsurface sediments to LDX 4-5 at depth, this would clearly be a diagenetic signal, however, this is not the case.Kl öcker et al. ( 2006) have also noted that, in their core 905 from the N.W. Indian Ocean, diagenesis has had minimal effect on the LDX record.The correlation of pteropod abundances in MIS 2 and MIS 6 across a range of oceans and environments also implies that the LDX profile is caused by a global climate signal and not merely by variations in local water chemistry.
Water chemistry around the Lesser Antilles island arc is however, complicated by influences of several water masses flowing between the islands and through a number of deeper passages into the Caribbean Sea (Peterson and Cofer-Shabica, 1987;Peterson, 1990, Broecker andClark, 2002;Sepulcre et al., 2009).Gerhardt and Henrich (2001) found that the influence of Antarctic Intermediate Water (AAIW), towards the south of the island arc, caused moderate to very poor preservation of Limacina inflata.However, towards the north of the island arc, the influence of AAIW is minor due to a large volume of Upper North Atlantic Deep Water (UNADW), which flows through the nearby Anegada Passage.This area consequently shows very good preservation of Limacina inflata.Gerhardt and Henrich (2001) place the aragonite saturation depth at 2000 m and the Aragonite Compensation Depth (ACD) at 3800 m water depth in this area.CAR-MON 2 was collected in 1102 m water depth, which is above the aragonite lysocline and ACD, thus discounting any effects that this may cause.A possible interference in the dissolution profile may be caused by inputs of volcanic ash, which can change the oceanic pH in the local area dramatically during and just after an eruption.A recent study has shown that, under laboratory conditions, volcanic materials entering sea water produce a significant change in pH (Jones and Gislason, 2008).This local impact on the pteropod fauna has been investigated and described elsewhere (Jones et al., 2009;Wall-Palmer et al., 2011).However, our observations suggest that, in this case, the ash from the South Soufri ère Hills volcano has had little or no effect upon the overall LDX profile.This is because the ash found within CAR-MON 2 is the result of several relatively short-lived events rather than one large, long-lasting event.Ash from these individual eruptions would have been so greatly diluted upon entering the ocean, that the acidic impact upon surface water fauna would have been insignificant.The assumption that the LDX profile is the result of changing oceanic pH is in agreement with recent laboratory work on living pteropods (Fabry et al., 2008;Comeau et al., Introduction Conclusions References Tables Figures

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Full 2009, 2010a, b) and pteropods from sediment traps in the Southern Ocean (Roberts et al., 2008).It also compares favourably with shell-weight data of Globigerina bulloides provided by recent work in the Southern Ocean (Barker and Elderfield, 2002;Moy et al., 2009).
Our results suggest that the distribution and abundance of shelled pteropod and heteropod fauna, and the quality of their preservation through the last 250 000 yr, provides a model for changes caused by climate variations.This signal appears to be worldwide and may help to predict future changes in the aragonitic holoplanktic fauna caused by increases in pCO 2 and the resulting changes in oceanic pH.However, since the level of anthropogenic CO 2 entering the oceans is now increasing at a rate 100 times faster than any changes seen in the past 650 000 yr (Fabry et al., 2008), it might be inappropriate to apply such a model to the modern oceans.The fate of the modern-day aragonitic holoplankton is uncertain, however, this study shows that, at oceanic pH levels relatively higher than those predicted for the 21st Century, euthecosome pteropods have been noticeably affected.Introduction

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Full Preservation of pteropod shells is, potentially, impacted by water chemistry during life, passage through the water column (probably minimal as they have quite high settling rates of 1-2.5 cm s −1 ; see Byrne et al., 1984), on the water/sediment surface and during burial.
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