Reconstructing the history of in a coastal

Caroline P. Slomp* Department of Earth Sciences–Geochemistry, Faculty of Geosciences, Utrecht University, 3508 TA, Utrecht, The Netherlands

Areas of the coastal where is low or absent in bottom Strong variations in the geochemical and paleo-ecological compo- waters, so-called dead zones, are expanding worldwide (Diaz and Rosen- sition and genetic signature of the in the provide berg, 2008). Increased inputs of from land are enhancing algal testimony that the conditions in the have been far from blooms, and the sinking of this organic matter to the seafl oor and subse- constant over the past ~7.5 kyr. Two phases of deposition are generally quent decay leads to a high oxygen demand in bottom waters. Depending distinguished based on visual characteristics of the sediments. Following on the physical characteristics of the coastal system, this may initiate peri- the onset of anoxia, a fi nely laminated, dark, organic-rich layer odic or permanent water column anoxia and euxinia, with the latter term formed fi rst (Unit II), followed by deposition of alternating microlami- implying the presence of free sulfi de (Kemp et al., 2009). Global warming nae of calcareous (white) and organic- and clay-rich material (black) from is expected to exacerbate the situation, through its effects on oxygen solu- ca. 2.6 kyr B.P. to the present (Unit I). The shift from Unit II to Unit I bility and water column stratifi cation. In many modern coastal systems, was originally attributed to the invasion of the Emiliania anthropogenic changes are superimposed on natural variation and lack of huxleyi when salinity rose above 11 (Arthur and Dean, 1998). However, knowledge of such variation makes the prediction of future changes in genetic analyses show that this calcifying haptophyte colonized the photic water column oxygen challenging (e.g., Grantham et al., 2004). That natu- zone of the Black Sea shortly after the connection to the Bosporus, and ral drivers alone can be the cause of widespread coastal anoxia is evident the Unit I–II transition marks the moment that coccoliths began to be pre- from studies of greenhouse periods in Earth’s past, including the oceanic served in the sediments (Coolen et al., 2009). anoxic events of the Cretaceous and Toarcian (Jenkyns, 2010). The delayed appearance of Unit II on the slopes of the basin has been Sediment proxy records are essential to any reconstruction of varia- taken as an indicator of a slow rise of the following the onset tions in anoxia and euxinia on time scales beyond several decades to a of anoxia (Degens and Ross, 1974). The rise was fast enough, however, for century. A variety of biological and geochemical indicators can be used the chemocline to reach the by the time of deposition of the for this purpose, such as the presence of the remains of benthic and pelagic lower part of Unit II, as indicated by the presence of biomarkers for pho- organisms, laminations, biomarkers for eukaryotes or prokaryotes, and tosynthetic (Sinnighe Damsté et al., 1993; Repeta, inorganic geochemical and mineralogical signatures in the sediment, and 1993). Results of similar analyses for the upper part of Unit II suggested a ideally, these methods are combined. Sediments that are deposited below subsequent descent of the chemocline followed by re-establishment in the a euxinic water column are, for example, typically enriched in organic photic zone during deposition of Unit I. At the time, controversy remained carbon, sulfur, iron, and trace metals such as rhenium and molybdenum about the temporal and spatial variability in the position of the chemo- (Gooday et al., 2009). Recent additions to this paleo- toolbox are the cline, and the extent to which the water column and photic zone remained isotope systems of Fe and Mo (Lyons et al., 2009). Reconstruction of the euxinic throughout deposition of Units I and II. This debate was partially temporal changes in the oxic-anoxic interface (chemocline) in the water resolved when Wilkin et al. (1997) showed that the size of the fram- column forms a key step in the identifi cation of the external drivers and boids in Units I and II were in line with a continuously euxinic water internal feedbacks that contribute to anoxia and euxinia in a given system. column. Using a composite record of sediment Fe, Mo, and Fe-isotopes In their study of sediments from the Black Sea, Eckert et al. (2013, p. 431 derived from data for nine sites throughout the basin, Eckert et al. (2013) in this issue of Geology), make this step by providing, for the fi rst time, a now confi rm the evolution of Black Sea euxinia, as suggested in these basin-wide reconstruction of the evolution of the chemocline in this silled earlier studies, and provide a more consistent and basin-wide timing for coastal basin over the Holocene. the series of events. Silled basins in humid areas such as Kau Bay (Indonesia), the Baltic The variation in strength of the ‘Fe shuttle’ forms the heart of their Sea, and the Black Sea, are particularly sensitive to low oxygen conditions reconstruction. This term is used to describe the lateral transfer of Fe because of salinity stratifi cation and associated reduced vertical mixing released from suboxic shelf sediments to the deep basin. The authors use (Kemp et al., 2009). All these inland have an intriguing history and their Fe/Al record as a direct indicator of the position of the chemocline, were originally coastal that were transformed to marine basins due where low Fe/Al indicates a weak shuttle with a chemocline impinging to postglacial sea-level rise. Kau Bay is only semi-euxinic, and is sub- on the slope. A high Fe/Al, in contrast, indicates a chemocline allowing ject to incursions of low-oxygen non-sulfi dic bottom waters that alternate suboxic water to spread over part of the shelf and supporting an intense with periods of anoxic, sulfi dic bottom waters (Middelburg et al., 1991). Fe shuttle. The authors also make use of the fact that Mo data can be used The Baltic Sea also alternates between redox states: it experienced vari- to reconstruct the of a basin, which for the Black Sea allows ous periods of low oxygen over the Holocene, but is currently subject to an estimate of the infl ow of Mediterranean . Fe isotope analyses a human-induced period of anoxia, with its bottom waters largely oxic bolster the argument for the shelf-source of Fe. The emerging timeline is around 1900 CE (Conley et al., 2009). The Black Sea is the largest euxinic as follows (Unit II): (1) a gradual rise of the chemocline over a period of basin in the world and differs in being permanently euxinic. This is the ~2 kyr following the onset of anoxia at ca. 7.6 kyr B.P., (2) fully devel- result of the strong stratifi cation that developed after its fore-runner fresh oped euxinic conditions with an ascent of the chemocline onto the shelf at water became connected to the Mediterranean Sea through the nar- ca. 5.3 kyr B.P., (3) a subsequent descent of the chemocline, and (Unit I) row, shallow Straits of the Bosporus at ca. 9 kyr B.P. Water column anoxia (4) establishment of the chemocline in its present-day position at the shelf developed across the deep basin from ca. 7.5 kyr B.P. onward (Degens break from 2.7 kyr B.P. onward. and Ross, 1974), and the chemocline is presently located at ~100 m depth. But this is not the full story. Besides a good timeline for euxinia in the Black Sea, we need to understand the hydrographic and biogeochemi- *E-mail: [email protected]. cal processes that drove these changes in redox conditions, and there much

GEOLOGY, April 2013; v. 41; no. 4; p. 523–524 | doi:10.1130/focus0420131.1 ©GEOLOGY 2013 Geological | April Society2013 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 523

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/41/4/523/3546634/523.pdf by guest on 29 September 2021 work still needs to be done. The evolution of the salinity in the basin, Coolen, M.J.L., 2011, 7000 years of Emiliana huxleyi viruses in the Black Sea: for example, is not well constrained. Recent qualitative reconstructions Science, v. 333, p. 451–452, doi:10.1126/science.1200072. of salinity based on various proxies suggest that values of surface water Degens, E.T., and Ross, D.A., 1974, The Black Sea—Geology, Chemistry, and Biology: American Association of Petroleum Geologists Memoir 20, 633 p. salinity in the Black Sea rose until ca. 3 kyr B.P., followed by a gradual Diaz, R.J., and Rosenberg, R., 2008, Spreading dead zones and consequences freshening to present-day values (van der Meer et al., 2008; Coolen, 2011). for marine ecosystems: Science, v. 321, p. 926–929, doi:10.1126/science Possible causes for the freshening include an increase in fl uvial discharge .1156401. and decreased evaporation (Giosan et al., 2012). An associated increase Eckert, S., Brumsack, H.-J., Severmann, S., Schnetger, B., and März, C., 2013, Establishment of euxinic conditions in the Holocene Black Sea: Geology, in stratifi cation may have contributed to the shallowing of the chemocline v. 41, p. 431–434, doi:10.1130/G33826.1. at the onset of the deposition of Unit I. Also, the processes leading to Giosan, L., Coolen, M.J.L., Kaplan, J.O., Constantinescu, S., Filip, F., Filipova- the increased total organic carbon (TOC) in Unit II as compared to the Marinova, M., Kettner, A.J., and Thom, N., 2012, Early anthropogenic overlying and underlying units are not well understood. The high TOC is transformation of the Danube-Black Sea system: Scientifi c Reports, v. 2, frequently interpreted as an indicator of enhanced availability and p. 582, doi:10.1038/srep00582. Gooday, A.J., Jorissen, F., Levin, L.A., Middelburg, J.J., Naqvi, S.W.A., Rabalais, following the infl ow of Mediterranean seawater, and transi- N.N., Scranton, M., and Zhang, J., 2009, Historical records of coastal eutro- tion of limnic (oxic) to marine (anoxic and euxinic) conditions (also see phication-induced : Biogeosciences, v. 6, p. 1707–1745, doi:10.5194 Eckert et al., 2013). However, the sources of the nutrients fuelling this /bg-6-1707-2009. productivity have not been identifi ed and whether, for example, phospho- Grantham, B.A., Chan, F., Nielsen, K.J., Fox, D.S., Barth, J.A., Huyer, A., Lub- chenco, J., and Menge, B.A., 2004, -driven nearshore hypoxia sig- rus release from sediments or river water is more important is still an open nals ecosystem and oceanographic changes in the northeast Pacifi c: Nature, question. Finally, the cause of the descent of the chemocline after ca. 5.3 v. 429, p. 749–754, doi:10.1038/nature02605. kyr B.P. remains unknown. While Eckert et al. (2013) propose a decreased Jenkyns, H.C., 2010, Geochemistry of oceanic anoxic events: Geochemistry Geo- seawater input or increased river input as potential causes, van der Meer physics Geosystems, v. 11, Q03004, doi:10.1029/2009GC002788. et al. (2008), in contrast, suggest that the absence of a shallow chemocline Kemp, W.M., Testa, J.M., Conley, D.J., Gilbert, D., and Hagy, J.D., 2009, Tem- poral responses of coastal hypoxia to nutrient loading and physical controls: can be best explained by the high sea-surface salinity at the time. Biogeosciences, v. 6, p. 2985–3008, doi:10.5194/bg-6-2985-2009. Despite the open questions, the Eckert et al. (2013) study is impor- Lyons, T.W., Anbar, A.D., Severmann, S., Scott, C., and Gill, B.C., 2009, Track- tant because it provides a more solid timeline and integrated view of the ing euxinia in the ancient ocean: A multiproxy perspective and Protorozoic evolution of euxinia in the Black Sea, which is highly useful for assess- case study: Annual Review of Earth and Planetary Sciences, v. 37, p. 507– 534, doi:10.1146/annurev.earth.36.031207.124233. ments of climatic and other drivers of temporal change. The tools used Middelburg, J.J., Calvert, S.E., and Karlin, R., 1991, Organic-rich transitional fa- can also be applied to better interpret sediment records from other marine cies in silled basins: Response to sea-level change: Geology, v. 19, p. 679– systems, both modern and ancient, and can thereby aid in the assessment 682, doi:10.1130/0091-7613(1991)019<0679:ORTFIS>2.3.CO;2. of the time scales of a possible decline into, and recovery from, wide-scale Repeta, D.J., 1993, A high-resolution historical record of Holocene anoxygenic anoxia and euxinia. Such knowledge is important in a warming world in the Black Sea: Geochimica et Cosmochimica Acta, v. 57, p. 4337–4342, doi:10.1016/0016-7037(93)90334-S. where water column deoxygenation in the coastal zone is becoming more Sinnighe Damsté, J.S., Wakeham, S.G., Kohnen, M.E.L., Hayes, J.M., and de Leeuw, and more common. J.W., 1993, A 6,000-year sedimentary molecular record of chemocline excur- sions in the Black Sea: Nature, v. 362, p. 827–829, doi:10.1038/362827a0. REFERENCES CITED van der Meer, M.T.J., Sannngiorgi, F., Baas, M., Brinkhuis, H., Sinninghe Dam- Arthur, M.A., and Dean, W.E., 1998, Organic-matter production and preserva- ste, J.S., and Schouten, S., 2008, Molecular isotopic and dinofl agellate evi- tion, and evolution of anoxia in the Holocene Black Sea: Paleoceanography, dence for Late Holocene freshening of the Black Sea: Earth and Planetary v. 13, p. 395–411, doi:10.1029/98PA01161. Science Letters, v. 267, p. 426–434, doi:10.1016/j.epsl.2007.12.001. Conley, D., et al., 2009, Hypoxia-related processes in the Baltic Sea: Environmen- Wilkin, R.T., Arthur, M.A., and Dean, W.E., 1997, History of water-column an- tal Science & Technology, v. 43, p. 3412–3420, doi:10.1021/es802762a. oxia in the Black Sea indicated by pyrite framboid size distribution: Earth Coolen, M.J.L., Saenz, J.P., Giosan, L., Trowbridge, N.Y., Dimitrov, P., Dimitrov, and Planetary Science Letters, v. 148, p. 517–525, doi:10.1016/S0012-821X D., and Eglinton, T.I., 2009, DNA and lipid molecular stratigraphic records of (97)00053-8. haptophyte succession in the Black Sea during the Holocene: Earth and Plan- etary Science Letters, v. 284, p. 610–621, doi:10.1016/j.epsl.2009.05.029. Printed in USA

524 www.gsapubs.org | April 2013 | GEOLOGY

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/41/4/523/3546634/523.pdf by guest on 29 September 2021