Can foraminifera be used as indicator of environmental changes?

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1 Can foraminifera be used as indicator of environmental changes? Richard Hofmann Technische Universität Bergakademie Freiberg Abstract. The analysis of foraminifera is a valuable utility in modern geosciences. As has been proved in the last decades, foraminifera are very usefull in tasks of environmental research. I would like to give an overview on some concepts, the way they work and the restriction of the applicability. In an effort to underline the acceptation of foraminifera in detection of environmantel changes, some examples from geological/environmental research will be given. 1. Introduction The aim of ecological studies is to prove the relationship between the biota and the environment. Due to the abundance and the formation of well preservable shells, foraminifera are very useful in analysing and assessing recent and ancient marine environments (Murray, 2002). The ecological behavior of foraminifera depends on physical, chemical and biological factors. The variation of any attribute, leads to a modification of behavior, chemism and metabolism of the individual specimen respectively the whole community. The purpose is, to attribute the change in living activities to actual environmental variations. 2. Concepts In order to subsume the versatile techniques in environmental research, I present three basic concepts to conclude the environmental setting and change by the help of the analysis of formanifera.

2 2 Richard Hofmann 2.1 Assemblage Analysis These methods are based on the quantitative and qualitative analysis of communities. Every species of foraminifera claims special demands to its environment. By summarizing the individual preferences of each species, it is possible to define a common ecological window for the considered attributes in the habitat. The ecological requirements for a species are extracted from recent studies on living specimens. Therefore, the applicability for ancient habitats requires that the same taxa prefer identic, at least similar conditions. Of course, the sole definition of the environmental attributes is not sufficient to note changes. Therefore, it is necessary to sample more than one area in order to put the differential ecological factors in spatial and temporal context. For instance, the micropalaeontological preparation of selected slices from sediment core yields different faunal distributions which can be used to reconstruct the development of palaeocommunities over one profile. Otherwise, those facies analyses could be used to distinguish sedimentary and palaeogeographic settings within one time interval. One of the main problems of the assemblage analysis is the distinction between natural variability and apparent environmental change (fig. 1). For instance, the natural variation caused by predation or seasonal fluctuations do not reflect disturbances of the considered ecosystem. The ocean is a 3-D living space. Therefore, it is obvious that planktic faunas are representing other ecological attributes than benthic communities. Foraminfera have colonized the free water column at least since the Jurassic (Oxford et al. 2002). For this reasons it is recommended to separate planktic and benthic taxa in reference to environmental conclusions. To avert the mixing of environmental signals, the determination of autochtonous and allochtonous faunal components is necessary. Likewise, the redeposition of already dead organisms, so called ghost-communities, could be lead to misinterpretation of the environment.

3 Can foraminifera be used as indicator of environmental changes? 3 Fig.1. Variability and change: (A) variability of a mean without change, (B) variability of a mean with a progressive change, (C) variability of a mean that has longer-term cyclic variation, segments a and b appear to indicate change (redrawn from Murray, 2000). 2.2 Measurements Foraminifera provide amazing proxy data, by chemical analyses and measurements. As a result of the formation of durable carbonatic shells, foraminifera are able to conserve the predominant ratio of stable isotops of Oxygen, Carbon and Calcium, at the time of their demise. The most important ratio of isotopes are 18 O/ 16 O and 13 C/ 12 C. The reasons for isotopic ratio variation are versatile (fig. 2). The palaeoecological attributes, which can assessed by isotope ratio are palaeotemperature, palaeosalinity, or ancient water supplies. Also in isotope analysis, it is strongly recommended to distinguish between planktic and benthic faunas (fig. 3). The stable isotope ratio is not constant within the water column. Already, the difference of the water temperature from bottom

4 4 Richard Hofmann waters to medium and shallow water, will have an significant influence on isotope data in foraminiferal tests. Fig. 2. Diagram illustrating how the stable isotopes of oxygen and carbon in microfossil tests will tend to vary with depth and salinity. The black arrows show the most typical isotope trends seen as environments become more extreme. (redrawn from Armstrong & Brasier 2005). Oxygen Isotopes (Armstrong & Brasier, 2005) Five main factors affect the ratio between the stable isotopes of oxygen in the carbonatic test of foraminifera. For the influence of one of these to be calculated, the other will be need to be estimated or known. a) isotopic composition of water There are many factors that shift the δ 18 O. High evaporation rates on water surface leads to an enrichment of 18 O. b) temperature Carbonates, built in warm water incorporate more 16 O than those precipitated in cooler water.

5 Can foraminifera be used as indicator of environmental changes? 5 c) mineral phase Aragonitic tests contain more 18 O than those composed of calcite. d) vital effects Due to metabolic processes, the secretion of CaCO 3 in living organisms is not in equilibrium with sea water. The amount of shift depends on the taxa and size. e) diagenesis The ratio could also be disturbed by meteoric waters and diagenetic fluids Fig. 3. Fundamental differences in δ 18 O between benthic and planktonic foraminifera (redrawn from Armstrong & Brasier 2005). Carbon Isotopes (Armstrong & Brasier, 2005) The 13 C/ 12 C-ratio is affected be several processes of isotope fraction, depletion or enrichment. a) surface water productivity At high primary productivity, 12 C is more likely removed from the ocean and atmosphere as the heavier 13 C. b) biological oxidation The respiration of organic matter on the ocean floor and in mid-waters leads to a re-release of 12 C to the water column. c) upwelling and mixing Where 13 C-depleted waters came up to the sea surface as by upwelling, the common ratio will be shifted.

6 6 Richard Hofmann d) carbon burial The deposition of large amounts of organic matter means a temporal deprivation of carbon isotopes and yields to an increase of the δ 13 C. e) diagenesis δ 13 C is much easily reset by meteoric and burial diagenesis than δ 18 O. The most fluids tend to carry 12 C. 2.3 Monitoring These methods are particularly used in modern and subrecent environments. The main technique consists on collecting living specimens and investigation of the biologic and environmental behavior. The goal is, to prove the impact of pollutants and salinity fluctuations on recent habitats. Foraminifera are suitable because of their short life cycle, their large numbers, and their ability to react quickly on environmental disturbance (Coccioni, 2000). An apparently response of foraminifera to pollutants and variations of salinity (Geslin et al. 2000) is the development of abnormal shell architectures. Different types of shell deformation are recognized. Reduced shell size, aberrant chamber shape, disturbed chamber arrangement, additional chambers, nondeveloped test and siamese twins are abundant in stressed enironments (Coccioni, 2000; Geslin et al. 2000). Similar to the technique of assemblage analysis (see 2.1), recent environments can easily assessed by the foraminiferal distribution. 3. Examples 3.1 The Messinian Salinity Crisis Blanc-Valleron et al. (2002) A section of the Tripoli Formation ( Ma) has been sampled for a high resolution integrated investigation on stable isotopes of carbonate, microfossils, and sedimentology. The Tripoli Formation of the Central Sicilian Basin provides good records of the environmental changes during the transition from the beginning of restrictive circumstances to the onset of the salinity crisis of the ancient Mediterranean. The Falconara/Gibliscemi sections are subdivided in 46 precession-controlled cycles.

7 Can foraminifera be used as indicator of environmental changes? 7 The workers summarized the distributional trends of the microfaunal assemblages, the signals of carbonate stable isotopes and the sedimentary record of each cycle, to show the transition from normal marine to hypersaline conditions. Beside other microfossils, such as radiolarians and diatoms, foraminifera play a significant role in this study. The authors assumed, several steps, in which the transition had proceed. Period 1: Large Atlantic-Mediterranean exchange. ( Ma) The most abundant species of foraminifera in this sequences are cosmopolitan, subtropical forms which indicate warm water and normal saline values. Another hint for a permanent exchange is documented by the incidental appearance of temperate north Atlantic forms. Period 2: increased restriction ( Ma) The presence of north Atlantic forms indicates water mass movement through the Mediterranean gateway. Although, the onset of poorer water exchange is shown by the composition of benthic foraminifera assemblages that indicate poorly ventilated bottom waters. Period 3: further restriction (6.29 Ma) The decrease of planktic foraminfera and the disappearance of Atlantic radialorians describe a distinct basin restriction. Furthermore, characteristic foraminifera species for stressed environments and higher salinities are recognized. This interval coincides with a worldwide glacio-eustatic sea level drop. Period 4: the isolation (<6.29 Ma) The final isolation of the Mediterranean from the world water circulation is found in the last cycles of the Tripoli-sequence. An evidence for hypersaline conditions is given by the complete absence of any marine microfossils. The transition culminates in the formation of evaporites and desiccation features recognized in the sedimentary record. 3.2 Changing deep water sources (Friedrich et al. 2004) Based on planktic and benthic foraminiferal stable isotope data, a global cooling during the Campanian/Maastrichtian has been suggested by several workers (Clarke and Jenkyns, 1999, Huber et. al., 2002). The authors suggest that variation of stable isotope data of benthic and planktic foraminifera reflecting changes in the circulation patterns and water formation in the former North Atlantic basin. Black Nose, the drilling side where the samples had extracted, was located at a paleolatitude of ~30 North (Hay et al. 1999) and a average paleo water depth of m (Norris et al. 1998). Therefore, Black Nose is

8 8 Richard Hofmann suitable to investigate intermediate to deep water mass movement and may be reflect changes of ancient deep water formation site. Under strong greenhouse conditions, oceanic circulation is driven by the sinking of warm, saline waters in low-latitude regions under excessive evaporation. Those waters have a characteristic isotope-ratio with high δ 18 O- and low δ 13 C- values. At the high North Atlantic, the deep water formation is a result of the sinking of dense, lower saline water. Waters like these tends to low δ 18 O- and high δ 13 C- values. The benthic foraminiferal tests, found at Black Nose, records a fluctiation of these stable isotope-ratio. To prove this assumption, grey values of the sediment has been measured. The variation of these values seem to fit the insolation fluctuation, generated by Milankovich cycles. During times of higher insolation, the deep water production in the North Atlantic was reduced as compared to cooler periods. During times of proposed low insolation, the reduced temperatures probably led to increased thermohaline deep water formation in the high North Atlantic. Therefore, the authors conclude, that the ratio of stable isotope in the shells of benthic foraminifera, represent the predominant mode circulation. 3.3 The indication of heavy metal pollution (Coccioni 2000) Benthic foraminiferal distribution and trace element concentration were investigated in surface sediments of several stations from the Goro Lagoon/Italy. The study area is suitable for studying the response of benthic foraminifera to trace metal pollution because of the strong levels of heavy metal pollution and the high proportion of abnormal tests. The study is based on 47 surface grab sediment time avereged samples collecteted in the Goro Lagoon. The author had analysed the faunal distribution, the mode of abnormality of tests and the concentration of trace metals. The aim was, to evaluate a correlation between morphologic abnormalities and the grade of pollution. Already, the living faunal distribution shows a significant influence of environmental stress. Only eight species were identified as living. The assemblages are largely dominated by 3 species. Therewith, the fauna fits a typical low diversity assmeblage. Any of the typical test deformities (fig. 5) are found in the investigated area. Due to the high leveled pollution with a various number of trace metals (V, Ni, Zn, Cr, Co, Pb, Cu, Hg, Th,) a direct correlation of the percentage of abnormal tests between the content and concentration of pollutants is not easily to show. Although, some relations could be approved. It has been shown that the percentage of abnormal specimens depends on lowered salinities in the Goro

9 Can foraminifera be used as indicator of environmental changes? 9 Lagoon. Furthermore, a clear positive correlation between concentration of Th and the percentage of abnormalities was found. Fig. 4. SEM photomicrographs showing examples of nondeformed and morphological abnormalities in benthic foraminifera. (redrawn from Geslin et al. 2000).

10 10 Richard Hofmann References Armstrong H. A., Brasier M. D. (2005). Microfossils. Second Edition. Blackwell Publishing Blanc-Valleron, M.M., Pierre, C., Caulet, J.P., Caruso, A. ; Rouchy, J.M., Cespuglio, G., Sprovieri, R., Pestrea, s. & Di Stefano, E. (2002). Sedimentary, stable isotope and micropaleontological records of the paleoceanographic change in the Messinian Tripoli Formation (Sicily, Italy). Palaeogeography, Palaeoclimatology, Palaeoecology 185, Clark; L.J:, Jenkyns, H.C. (1999). New oxygen isotope evidence for long-term Cretaceous climatic change in the Southern hemisphere. Geology 8, Coccioni R. (2000). Benthic Foraminifera as bioindicators of heavy metal pollution. Environmental Micropaleontology, Volume 15 of Topics in Geobiology. Kluwer Academic/Plenum Publishers, New York Friedrich O., Herrle J.O., Klößer P., Hemleben C. (2004). Early Maastrichtian stable Isotops: changing deep water sources in the North Atlantic? Palaeogeography, Palaeoclimatology, Palaeoecology 211, Geslin E., Stouff V., Debany J. P., Lesourd M. (2000). Environmental variation and foraminiferal test abnormalities. Environmental Micropaleontology, Volume 15 of Topics in Geobiology. Kluwer Academic/Plenum Publishers, New York, Hay, W.W., DeCONto, R.M., Wold, C.N., Wilson, K.M., Voigt, S., Schulz, M., Rossby Wold, A., Dullo, W.C., Ronov, A.B., Balukhovsky, A.N., Söding, E. (1999) Alternative global Cretaceouspaleogeography. In: Barrera, E., Johnson, C.C. (Eds). Evolution of the Cretaceous Ocean-Climate System. Special Paper-Geological Society of America, vol 332, pp Boulder Huber, B.T., Norrris, R.D. MacLeod, K.G., (2002) Deep-sea paleotemperature record of extreme warmth during the Cretaceous. Geology 30, Murray, J.W. (2000). When does environmental variability become environmental change? The Proxy Record of benthic Formanifera. Environmental Micropaleontology, Volume 15 of Topics in Geobiology. Kluwer Academic/Plenum Publishers, New York, Norris, R.D., Kroon, D., Klaus, A., et al. (1998). Proceedings of the Ocean Drilling Program. Initial Reports, vol. 171B. Ocean Drilling Program Celloge Staion, TX, Oxford, M.J., Gregory, F.J., Hart, M.B., Henderson, A.S., Simmons, M.D. & Watkinson, M.P. (2002). Jurassic planktonic foraminifera from the United Kingdom. Terra Nova 14,