Lake Iznik, situated in the Marmara region (NW Turkey), is an alkaline lake with about 300 km2 surface area, inserted in an area of typical Mediterranean climate. During the dry summer season, carbonates are precipitating from the water column. The endogen carbonate accumulation, e.g. aragonite, is expected to hold past climate information. A detailed understanding of the limnological system is required to differentiate site specific signals and responses to climatic forcing.
Geochemical and mineralogical evidence from a continuous composite profile was documented in a decadal to centennial time scale. A novel improved age model shows that the sediment record reaches up to ~ 31.5 ka cal BP. Compositional Data (CoDa) analysis allowed the identification of three groups of elements, and particular elements with distinct geochemical behavior, which indicate specific geochemical processes. Also, CoDa analysis allowed clear geochemical characterization of litho-stratigraphic units.
Grain size measurements were undertaken to determine the energy levels of the physical deposition environment. Detailed grain size analysis allowed amorphous silica to be quantified. From siliciclastic grain size analysis, the ratio coarse silt/clay was identified to be a sensitive indicator to infer changes in the water column depth. This ratio was calibrated to the modern depositional regime.
The endogen carbonate production proved to be a sensitive climatic indicator. Changes in carbonate concentration are timely associated to the inferred fluctuations in water column depth. The aragonite concentrations are most likely related to regional temperature, catchment hydrology and the mixing dynamics of the lake.
The physical mixing dynamics of the lake is reflected in (a) behavior of elements mobile under oxic/anoxic conditions, and (b) geochemical patterns for carbonate bound elements in hardwater lakes, and (c) stability of various minerals.
This study established the current knowledge of the geochemical evolution of Lake Iznik. It further adds to the understanding of paleoclimate evolution in the Marmara region on a millennial time scale.
From ~31 ka cal BP until the deglaciation at ~18 ka cal BP, Lake Iznik is characterized by low productivity and higher detrital load, in association to a low carbonate accumulation. Thicker epilimnion and lower supersaturation states are inferred in association to a deeper water column. During the last glacial, i.e. from ~ 26 ka cal BP to ~18 ka cal BP, Lake Iznik passes through prolonged stages of incomplete mixing of the water column, whereas the lake level is most likely maintained during most of the last glacial. In addition, the crystal structure of carbonates reflects mineral instability. At the Last Glacial Maximum (~ 22 ka cal BP) carbonate accumulation in the lake is nearly absent.
During the deglaciation, starting at ~18 ka cal BP, dynamic and pronounced lake level variations occur. A shallow water column is inferred at 16.5 ka calBP, and a possible low stand is identified for the period between ~ 14 and ~ 9 ka cal BP. Generally Marine Isotope Stage (MIS) 1 is marked by increased aragonite concentrations, and enhanced chemical weathering. The terrestric organic load increases gradually, and is accompanied by lake trophic conditions.
Lake Iznik climate event stratigraphy highly correlates with the regional geological record. The endogen carbonate accumulation seems to occur in phase to Northern Hemisphere climate variability, for instance warm interstadials and cold stadials are depicted. In synchrony with Dansgaard-Oeschger interstadials Iznik sediments point to a deep water column, which decreases in time. After a short time lag, geochemistry reacts with increasing aragonite concentrations. In synchrony with Heinrich stadials, the Iznik sediments underwent a short phase of magnesian calcite preservation, after that generally more elevated calcite concentrations prevail. In general the cold phases are associated to a higher input of detrital calcite, likewise during the Younger Dryas cold event (~ 12 ka cal BP).
The early Holocene (from ~12 to ~9 ka cal BP) is characterized by pronounced summer stratification and higher epilimnion carbonate supersaturation. Recurrent stages of good lake mixing are accompanying a shallow water column. The middle Holocene is generally more humid, as indicated by enhanced chemical weathering and by two distinct lake level increases. The first of such events occurs at circa 9.3 ka cal BP and is related to the reconnection of the Black Sea to Mediterranean water ways. This geological event is followed by the settlement of the first farming communities in the Iznik basin. Moreover, at circa 5 ka cal BP a change occurs in delivery of grain size proportions – in relation to what is expected according to the inferred water column depth. This is most likely resulting from anthropic land-use within the basin.