New insights into Holocene hydrology and temperature from lipid biomarkers in western Mediterranean alpine wetlands

Abstract

Alpine regions of the Mediterranean realm are among the most climatically sensitive areas in the world. Thus, alpine wetlands from the southern Iberian Peninsula, in the westernmost part of the Mediterranean region, are highly sensitive sensors of environmental changes. Difficulties have surfaced in separating controls by temperature and/or precipitation in previous paleoenvironmental studies from alpine environments in this area. We present a Holocene biomarker record (n-alkanes and long-chain diols) from a high elevation lake, Laguna de Río Seco (LdRS), in the south of the Iberian Peninsula, which contributes to the identification of these forcing mechanisms. The hydrological history of the area, primarily water availability and evapotranspiration, is reconstructed by means of the n-alkane record, including the indices of average chain length, portion aquatic, and carbon preference index, as well as hydrogen isotopes (δD) of aquatic (δD aq ) and terrestrial (δD wax ) n-alkanes. Temperatures are also estimated using the algae derived long-chain diols. We interpret δD aq and δD wax fluctuations as showing changes in the source and amount of precipitation throughout the LdRS record. An Atlantic precipitation source appears to have predominated during the early-middle Holocene, but an occasional Mediterranean influence with an isotopic enrichment in precipitation is detected in the middle-late Holocene that is likely related to the setting of the current atmospheric pattern in southeastern Iberia under the joint control of the North Atlantic Oscillation (NAO) and the Western Mediterranean dynamics, such as the Western Mediterranean Oscillation (WeMO). Our new record from LdRS is consistent with a generalized trend of a humid early-middle Holocene with low temperature variability, evolving towards an arid middle-late Holocene with abrupt temperature changes. In addition to these long-term trends during the last ∼10,500 years, two phases of climate instability, evidenced by abrupt depletions in δD aq , have been identified at the end of these periods, one between ~6500 and 5500 cal. yr BP and another in the last ~500 years. These episodes would represent strengthened winter cold conditions that favoured the persistence of snowpack and frozen soil in the catchment, causing reduced terrestrial plant growth and low lake evaporation. According to the long-chain diol record, temperatures during these phases were relatively low, but experienced abrupt increases at the end of each period.

The potential of gypsum speleothems for paleoclimatology: application to the Iberian Roman Humid Period

“…Also, the d-excess values in rainfall from Mediterranean convective events are normally higher than in precipitation from the Atlantic moisture source 28 . Changes in the contributions of the Mediterranean and Atlantic moisture sources to the total hydrological budget in SE Iberia have been proposed previously to explain the δD variability of n-alkanes in sediment profiles of alpine lake of the Sierra Nevada 26 . This mechanism could have partially influenced the isotopic composition dripwater in Sima Blanca Cave; however, the fact that the δ 18 O-δD values of the speleothem-forming water are linearly correlated, with a slope much lower than that of the Local Meteoric Water Line (Fig.…”

“…Main regional paleoclimate records mentioned in this study. This includes Sima Blanca cave (SBL cave, this study); Lake Zoñar [30][31][32][33] ; alpine lakes of Sierra Nevada 26,[34][35][36][37][38]41,47 and Alboran Sea sediment cores, ODP Site 976 and 300G 32 . or detrital contributions from the soil above the cave.…”

“…Convective rainfall events from the Mediterranean are isotopically enriched, and prevail during summer and autumn and during warm climatic periods. In contrast, rainfall with lower isotopic values are transported along Atlantic storm tracks, which dominate during winter and during colder climatic periods 25,26 The mean δ 18 O values of modern rainfall from the Mediterranean have been found to be up to 3‰ higher in this region than rainfall events from Atlantic fronts 27 . Also, the d-excess values in rainfall from Mediterranean convective events are normally higher than in precipitation from the Atlantic moisture source 28 .…”

“…Changes in the isotopic composition of rainfall and, therefore, infiltration water to the cave may also influence the SBL stalactite record. There are two main sources of humidity that may influence the isotopic composition of rainfall in the Mediterranean part of the Iberian Peninsula 25,26 . Convective rainfall events from the Mediterranean are isotopically enriched, and prevail during summer and autumn and during warm climatic periods.…”

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Figure 1 Main regional paleoclimate records mentioned in this study. This includes Sima Blanca cave (SBL cave, this study); Lake Zoñar 30 – 33 ; alpine lakes of Sierra Nevada 26 , 34 – 38 , 41 , 47 and Alboran Sea sediment cores, ODP Site 976 and 300G 32 . Figure created by InkScape 0.92.4 ( https://inkscape.org ).

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