Trans-Mediterranean comparison of geochemical paleoproductivity proxies in a mid-Pleistocene interrupted sapropel

https://doi.org/10.1016/j.palaeo.2005.03.020Get rights and content

Abstract

Sapropel layers in the sediment record of the Mediterranean Sea reflect periods of climate-induced elevated marine production associated with precessional minima. Some of these layers feature interruptions that represent centennial-to-millennial returns to the predominantly low-productivity conditions of this sea. We have measured multiple geochemical paleoproductivity proxies in the interrupted sapropel layer that corresponds to insolation cycle 90 (955 ka) at 1-cm intervals in a four-site transect from the Balearic Basin to the Levantine Basin to assess similarities and differences in its expression at the four locations. The interrupted sapropels are similarly divided by an organic-carbon-poor interval into an upper layer in which organic carbon concentrations reach 2–7 wt.% and a lower layer in which they peak between 1 and 3 wt.%. The interruption was essentially synchronous in the Balearic Basin, the Tyrrhenian Basin, and the Ionian Basin, but it was delayed and shorter in the Levantine Basin. Lighter nitrogen isotope compositions consistently accompany higher organic carbon accumulation rates in the sapropel layers, which implies a shift from coccolith-dominated organic matter production towards a mode of primary production in which cyanobacteria were important during times of sapropel deposition. Organic carbon isotopic compositions become heavier as organic carbon accumulation increases at three of the locations, which is consistent with elevated productivity. However, they become lighter at the Levantine Basin site, which suggests greater delivery of isotopically light fluvial waters to this part of the Mediterranean Sea. The local paleoclimate conditions conducive to sapropel formation evidently were not completely uniform across the Mediterranean region but their occurrences were synchronous. Moreover, the co-existence of an essentially same-age interruption of the sapropels at the four locations shows that climate could change rapidly and concordantly across the region at much less than precessional intervals.

Introduction

Multiple layers of sapropels, which are colored greenish-brown to black, enriched in organic carbon, and laminated, are characteristic of the Late Miocene to Quaternary sediment record of the Mediterranean Sea (e.g., Cramp and O'Sullivan, 1999). The sapropels contrast markedly against the light-colored, organic-carbon-poor homogeneous marls that comprise most of the sediment record and that typify modern sediments. Their repetitive appearance corresponds to precessional minima, which are times of northern hemisphere maximum seasonality that recur at ∼21 ky intervals. This feature indicates that the sapropels record times of globally forced paleoclimate and paleoceanographic changes that were amplified in the Mediterranean region. Of particular importance, the high concentrations of organic carbon in the sapropels imply some combination of elevated marine productivity and enhanced preservation of organic matter, neither of which exists under the modern conditions in this sea.

Its near-isolation from the world ocean and predominantly arid climate make the modern Mediterranean Sea a highly oligotrophic system. The anti-estuarine circulation of the Sea is driven by evaporation of its surface waters and seasonal cooling that combine to form dense water masses that sink from the surface to carry nutrients away from the euphotic zone and to ventilate deep basins. Primary production is weakly sustained by delivery of fresh nutrients from continental runoff (Bethoux, 1989). The precessional minima with which sapropels coincide were periods of wetter climate in the Mediterranean region (Rossignol-Strick, 1983, Rohling and Hilgen, 1991, Wehausen and Brumsack, 2000). The wetter climate increased river runoff, which delivered more soil-derived nutrients to the Mediterranean Sea (e.g., Sarmiento et al., 1988), thereby initiating greater primary production, and which simultaneously diluted its surface waters (e.g., Bernasconi and Pika-Biolzi, 2000), thereby increasing near-surface stratification and slowing the anti-estuarine circulation. Increased primary production would have been sustained by enhanced regeneration of phosphate from organic matter in bottom sediments during sulfate reduction (Slomp et al., 2002, Slomp et al., 2004). Equally important, the near-surface stratification could have encouraged cyanobacterial nitrogen fixation, which would have provided a ready supply of bioavailable nitrogen (e.g., Struck et al., 2001).

This scenario of climate-induced eutrophication would potentially be sensitive to place-to-place variations in increased precipitation, enhanced land run-off, and strengthened surface stratification. Because some sapropel layers exhibit multi-century interruptions that record excursions from the conditions that favored their formation (Milder et al., 1999, Myers and Rohling, 2000, Krishnamurthy et al., 2000, Mercone et al., 2001, Arnaboldi and Meyers, 2003, Casford et al., 2003), temporal variations in climatic conditions were also relatively common. Such interrupted sapropels—sometimes also called “composite” sapropels (Calvert and Fontugne, 2001)—present special opportunities to explore temporal and spatial variations in sapropel formation. In this contribution, we examine elemental and isotopic proxies of organic matter production and preservation in same-age mid-Pleistocene sapropel layers from four locations, and we discuss the results in terms of regional differences in the paleoclimatic processes that magnified marine productivity.

Section snippets

Settings and sampling

We obtained sediment sequences from Ocean Drilling Program (ODP) Sites 967, 969, 974, and 975 (Fig. 1). These sites represent different deepwater depositional settings in the Mediterranean Sea. Site 967 is on the flank of the Eratosthenes Seamount in the Levantine Basin. Although the nearest land is Cyprus to the north, this location is strongly influenced by variations in Nile River outflow and is hence sensitive to the paleoclimate fluctuations driven by the West Asian monsoon. Site 969 is on

Calcium carbonate, organic carbon, and total nitrogen concentrations

The sediment samples were freeze-dried and then ground to a homogeneous powder in an agate mortar and pestle. Concentrations of calcium carbonate were measured using the “Carbonate Bomb” procedure of Müller and Gastner (1971) in which a known weight of sediment is treated with 3N HCl to release a volume of CO2 that is proportional to its carbonate content. The carbonate-free residue from this procedure was recovered by centrifugation, rinsed to remove chlorides, and dried for subsequent

Results and discussion

The results of our analyses of same-age sapropels from the four locations show differences that have important implications about the paleoclimatic and paleoceanographic processes involved with sapropel formation.

Summary and conclusions

The sapropel layers corresponding to insolation cycle 90 at ODP Site 967 in the Levantine Basin, Site 969 south of Crete, Site 974 in the Tyrrhenian Basin, and Site 975 in the Balearic Sea display similar increases in organic carbon mass accumulation rates and decreases in δ15N values. The dramatic excursions to light δ15N values in the sapropel layers indicate that nitrogen-fixing photoautotrophic bacteria were very important during times of sapropel deposition. The changes to wetter climate

Acknowledgments

We thank Caroline Slomp and John Thomson for their thoughtful and very useful suggestions to improve this manuscript. Stephan Kim, Amy Boetcher, and Laurie Cotsonika measured the inorganic and organic carbon concentrations. The Ocean Drilling Program, funded by the National Science Foundation and IPOD countries, provided the samples and the special opportunity for PAM to spend two months in the western Mediterranean Sea during Leg 161.

References (65)

  • N. Hassold et al.

    Grain-size evidence of the delivery mode of terrigenous sediment components to a middle Pleistocene interrupted sapropel from ODP Site 969, Mediterranean Ridge

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (2003)
  • J.M. Hayes

    Factors controlling 13C contents of sedimentary organic compounds: principles and evidence

    Marine Geology

    (1993)
  • D. Mercone et al.

    High-resolution geochemical and micropaleontological profiling of the most recent eastern Mediterranean sapropel

    Marine Geology

    (2001)
  • P.A. Meyers

    Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic process

    Organic Geochemistry

    (1997)
  • M. Minagawa et al.

    Nitrogen isotope ratios of red tide organisms in the East China Sea: a characterization of biological nitrogen fixation

    Marine Chemistry

    (1986)
  • P.G. Myers et al.

    Modeling a 200-yr interruption of the Holocene sapropel S1

    Quaternary Research

    (2000)
  • A. Negri et al.

    Are productivity and stratification important to sapropel deposition? Microfossil evidence from late Pliocene i-cycle 180 at Vrica, Calabria

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (2003)
  • I.A. Nijenhuis et al.

    Geochemical constraints on Pliocene sapropel formation in the eastern Mediterranean

    Marine Geology

    (2000)
  • H.A. Paul et al.

    Oxygen isotopic composition of the Mediterranean Sea since the last glacial maximum: constraints from pore water analysis

    Earth and Planetary Science Letters

    (2001)
  • J. Rinna et al.

    Combined organic and inorganic geochemical reconstruction of paleodepositional conditions of a Pliocene sapropel from the eastern Mediterranean Sea

    Geochimica et Cosmochimica Acta

    (2002)
  • C.P. Slomp et al.

    Enhanced regeneration of phosphorus during formation of the most recent eastern Mediterranean sapropel (S1)

    Geochimica et Cosmochimica Acta

    (2002)
  • C.P. Slomp et al.

    Controls on phosphorus regeneration and burial during formation of eastern Mediterranean sapropels

    Marine Geology

    (2004)
  • S. Stefanelli et al.

    Foraminiferal record and environmental changes during the deposition of the Early–Middle Pleistocene sapropels in southern Italy

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (2005)
  • U. Struck et al.

    Biological productivity during sapropel S5 formation in the eastern Mediterranean Sea: evidence from stable isotopes of nitrogen and carbon

    Geochimica et Cosmochimica Acta

    (2001)
  • S.C. Twichell et al.

    Significance of high C / N ratios in organic-carbon-rich Neogene sediments under the Benguela Current Upwelling System

    Organic Geochemistry

    (2002)
  • B.A.S. van Mooy et al.

    Impact of suboxia on sinking particulate organic carbon: enhanced carbon flux and preferential degradation of amino acids via denitrification

    Geochimica et Cosmochimica Acta

    (2002)
  • X.-C. Wang et al.

    Stable carbon and nitrogen isotopic compositions of high molecular weight dissolved organic matter from four U.S. estuaries

    Geochimica et Cosmochimica Acta

    (2004)
  • R. Wehausen et al.

    Cyclic variations in the chemical composition of eastern Mediterranean Pliocene sediments: a key for understanding sapropel formation

    Marine Geology

    (1999)
  • R. Wehausen et al.

    Chemical cycles in Pliocene sapropel-bearing and sapropel-barren eastern Mediterranean sediments

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (2000)
  • D. Archer

    Modeling the calcite lysocline

    Journal of Geophysical Research

    (1991)
  • D. Archer et al.

    Effect of deep-sea sedimentary calcite preservation on atmospheric CO2 concentration

    Nature

    (1994)
  • Arnaboldi, M., Meyers, P.A., in press. Paleoclimate patterns across the Mediterranean Sea from organic carbon and...
  • Cited by (40)

    • Ichnological evidence for bottom water oxygenation during organic rich layer deposition in the westernmost Mediterranean over the Last Glacial Cycle

      2022, Marine Geology
      Citation Excerpt :

      In the Eastern Mediterranean Sea, the simultaneous occurrence of high productivity along with low oxygenation in the bottom waters is held to be the main factor behind sapropel deposition (e.g., Rohling et al., 2015). In the Western Mediterranean, previous work has evidenced that the main factors controlling ORL deposition are productivity and bottom water oxygenation, based on foraminifera, biomarker, and geochemical analysis (Casford et al., 2003; Meyers and Arnaboldi, 2005; Pérez-Asensio et al., 2020, among others). Debate persists about which causes are predominant, or if all events could have the same set of conditions (Rohling et al., 2015).

    • Deglacial to Holocene environmental changes in the northern Ligurian Sea: The dual influence of regional climate variability and large-scale intermediate Mediterranean circulation

      2021, Palaeogeography, Palaeoclimatology, Palaeoecology
      Citation Excerpt :

      Thus, modifications in the intermediate-deep water circulation are necessarily involved to promote sapropel deposition (Rogerson et al., 2008; Grimm et al., 2015; Rohling et al., 2015). In the western Mediterranean Sea (WM Sea), ORL were also associated to both enhanced productivity presumably related to increased continental runoff (Meyers and Arnaboldi, 2005), and reduced deep ventilation leading to low oxygen content and high preservation of organic matter at the sea bottom (Rogerson et al., 2008). Most of ORL and sapropels are expected to be deposited at the same time, because the intermediate water masses formed in Levantine basin (LIW) convey eastern Mediterranean salinity reduction into the western Mediterranean (Murat, 1999) enhancing the water column stratification.

    • Paleoclimate and paleoceanography over the past 20,000yr in the Mediterranean Sea Basins as indicated by sediment elemental proxies

      2015, Quaternary Science Reviews
      Citation Excerpt :

      This has been traditionally interpreted by invoking a northward migration of the Inter-tropical Convergence Zone (ITCZ) and intensified monsoonal activity in northern Africa, associated with insolation maxima during astronomical precession minima (e.g., Rohling et al., 2002; Liu et al., 2012). More intense rainfall induced lower sea surface salinity and higher nutrient input that also caused a change in the bacterial community, with the establishment of N-fixing bacteria (e.g., Meyers and Arnaboldi, 2005; Gallego-Torres et al., 2011). The bacterial bloom of an N-fixing community would have also sustained high diatom productivity (Kemp and Villareal, 2013) and maintained a general high productivity level, which led to a high accumulation rate of organic matter in the sediment with an ultimately total oxygen consumption in deep waters (e.g., Jilbert et al., 2010).

    View all citing articles on Scopus
    View full text