Gonzalo Jiménez-Moreno

Isotopic evidence of C4 grasses in southwestern Europe during the Early Oligocene–Middle Miocene

C 4 plants are widely successful in the grass-dominated ecosystems of tropical, subtropical, and warm-temperate regions, largely as a result of their ability to limit photorespiration and improve water-use efficiency. A widely held paradigm is that low ( 2 concentrations were an important factor selecting for the origin of C 4 plants, although support in geological records is limited. We determined the carbon isotopic composition of 686 individual grass-pollen grains preserved in eight samples of lacustrine and shallow-marine sediments from three basins spanning the Early Oligocene to Middle Miocene in southwestern Europe. Grasses composed 4 grasses. Thus C 4 grasses occurred on the landscape as early as the earliest Oligocene, ∼14 m.y. earlier than previous isotopic evidence of first C 4 plants and before p CO 2 fell during the Oligocene.

Anthropogenic impact and lead pollution throughout the Holocene in Southern Iberia.

Present day lead pollution is an environmental hazard of global proportions. A correct determination of natural lead levels is very important in order to evaluate anthropogenic lead contributions. In this paper, the anthropogenic signature of early metallurgy in Southern Iberia during the Holocene, more specifically during the Late Prehistory, was assessed by mean of a multiproxy approach: comparison of atmospheric lead pollution, fire regimes, deforestation, mass sediment transport, and archeological data. Although the onset of metallurgy in Southern Iberia is a matter of controversy, here we show the oldest lead pollution record from Western Europe in a continuous paleoenvironmental sequence, which suggests clear lead pollution caused by metallurgical activities since ~3900 cal BP (Early Bronze Age). This lead pollution was especially important during Late Bronze and Early Iron ages. At the same time, since ~4000 cal BP, an increase in fire activity is observed in this area, which is also coupled with deforestation and increased erosion rates. This study also shows that the lead pollution record locally reached near present-day values many times in the past, suggesting intensive use and manipulation of lead during those periods in this area.

Early and Middle Miocene dinoflagellate cyst stratigraphy of the Central Paratethys, Central Europe

Marine organic-walled dinoflagellate cysts have been studied from the Early and Middle Miocene of the Central Paratethys in Austria (Vienna and eastern Alpine Foreland basins) and Hungary (Pannonian Basin), and compared with assemblages of similar age from the Atlantic Ocean and Mediterranean Sea. The presence of a diverse flora of 71 taxa, including such biostratigraphical markers as Apteodinium spiridoides, Cerebrocysta poulsenii, Cordosphaeridium cantharellus, Cribroperidinium tenuitabulatum, Exochosphaeridium insigne, Glaphyrocysta reticulosa s.l., Habibacysta tectata, Labyrinthodinium truncatum subsp. truncatum, Palaeocystodinium miocaenicum, and Unipontidinium aquaeductus, has allowed the establishment of five biozones that characterize the Ottnangian, Badenian and Sarmatian local stages (collectively equivalent to the mid-Burdigalian, upper Langhian and Serravallian stages). This is the first study to demonstrate the applicability of dinoflagellate cysts for detailed stratigraphic correlation and palaeoenvironmental interpretation in the Early and Middle Miocene of the Central Paratethys area.

A high-resolution record of climate, vegetation, and fire in the mixed conifer forest of northern Colorado, USA

High-resolution pollen, charcoal, δ13C, total organic carbon (TOC), and magnetic susceptibility data from sediment cores from a montane lake in northern Colorado record variations in vegetation, fire history, and sedimentation since 14.5 ka (1 ka = 1000 cal yr B.P.). This record shows warm conditions during the Bolling-Allerod and the coldest conditions in this area during the Younger Dryas event. Warming occurred throughout the early and middle Holocene, lasting until ca. 5 ka, when the warmest and wettest summer conditions were recorded. Progressive climate cooling and enhanced winter precipitation are then observed until present day. These long-term climatic trends correlate to changes in summer insolation. Charcoal accumulation rates (CHAR) increased along with the arboreal vegetation, from minima in the Late Glacial period to maxima during the early and middle Holocene, suggesting that charcoal influx was also controlled by climate and vegetation. TOC and δ13C show a progressive increase and a decrease trend during the late Pleistocene and Holocene, respectively, related to changes in vegetation and productivity in the lake. Major peaks in the CHAR record correspond with peaks in magnetic susceptibility, indicating enhanced fire-induced erosion and sedimentation. Millennial- and centennial-scale changes are also observed throughout the different proxy records. They exhibit strong correlations with climate records from distant regions, including Greenland and the North Atlantic, providing evidence for global teleconnections among regional climates. A solar-climate connection is suggested by prominent ca. 225 and 390 yr cycles, which may correlate with the 208 yr (Suess) and 400 yr solar cycles.

Vegetation, sea-level, and climate changes during the Messinian salinity crisis

The Messinian salinity crisis (late Miocene) is one of the most fascinating paleoceanographic events in the recent geological history of the Mediterranean Sea, defining a time when it partly or nearly completely dried out. However, the relative roles of tectonic processes and sea-level changes, as triggers for restriction and isolation of the Mediterranean Sea from the open ocean, are still under debate. In this study, we present a detailed pollen, dinoflagellate cyst (dinocyst), and magnetic susceptibility analysis of a sequence of late Neogene (between ca. 7.3 and 5.2 Ma) marine sediments from the Montemayor-1 core (lower Guadalquivir Basin, southwestern Spain), which provides a continuous record of paleoenvironmental variations in the Atlantic side of the Betic corridors during the late Miocene. Our results show that significant paired vegetation and sea-level changes occurred during the Messinian, likely triggered by orbital-scale climate change. Important cooling events and corresponding glacio-eustatic sea-level drops are observed in this study at ca. 5.95 and 5.75 Ma, coinciding with the timing and duration of oxygen isotopic events TG32 and TG22–20 recorded in marine sediments worldwide. It is generally accepted that the onset of the Messinian salinity crisis began at ca. 5.96 ± 0.02 Ma. Therefore, this study suggests that the restriction of the Mediterranean could have been triggered, at least in part, by a strong glacio-eustatic sea-level drop linked to a climate cooling event occurring at the time of initiation of the Messinian salinity crisis.

Medieval warming initiated exceptionally large wildfire outbreaks in the Rocky Mountains

Significance In the western United States and other forested regions, climate change may increase both the frequency of wildfires and the amount of area burned. Studies of past climate changes and their effects on wildfires can provide constraints on potential future wildfire risks. Here, we reconstruct the history of wildfire across a representative subalpine forest landscape in northern Colorado over the past two millennia. Warming of ∼0.5 °C ∼1,000 years ago increased the percentage of our study sites burned per century by ∼260% relative to the past ∼400 y. The large increase in the number of sites burned by fires highlights the risk that large portions of individual landscapes may burn as climates continue to warm today. Many of the largest wildfires in US history burned in recent decades, and climate change explains much of the increase in area burned. The frequency of extreme wildfire weather will increase with continued warming, but many uncertainties still exist about future fire regimes, including how the risk of large fires will persist as vegetation changes. Past fire-climate relationships provide an opportunity to constrain the related uncertainties, and reveal widespread burning across large regions of western North America during past warm intervals. Whether such episodes also burned large portions of individual landscapes has been difficult to determine, however, because uncertainties with the ages of past fires and limited spatial resolution often prohibit specific estimates of past area burned. Accounting for these challenges in a subalpine landscape in Colorado, we estimated century-scale fire synchroneity across 12 lake-sediment charcoal records spanning the past 2,000 y. The percentage of sites burned only deviated from the historic range of variability during the Medieval Climate Anomaly (MCA) between 1,200 and 850 y B.P., when temperatures were similar to recent decades. Between 1,130 and 1,030 y B.P., 83% (median estimate) of our sites burned when temperatures increased ∼0.5 °C relative to the preceding centuries. Lake-based fire rotation during the MCA decreased to an estimated 120 y, representing a 260% higher rate of burning than during the period of dendroecological sampling (360 to −60 y B.P.). Increased burning, however, did not persist throughout the MCA. Burning declined abruptly before temperatures cooled, indicating possible fuel limitations to continued burning.

Impact of late-Holocene aridification trend, climate variability and geodynamic control on the environment from a coastal area in SW Spain

A detailed pollen analysis has been carried out on two sediment cores taken from a marsh area located in the Doñana National Park, southwestern Spain. The studied sedimentary sequences contain a similar late Holocene record of vegetation and climate and show a progressive aridification trend since at least 5000 cal. yr BP, through a decrease in forest cover in this area. Long-term vegetation changes shown here (semi-desert expansion and Mediterranean forest decline) paralleled declining summer insolation. Decreasing summer insolation most likely impacted negatively on tree growing season as well as on winter precipitation in the area. Superimposed on the long-term aridification trend were multi-centennial scale periods characterized by forest reductions or increases in arid and halophytic plants that can be interpreted as produced by enhanced droughts and/or by local geodynamic processes. These are centered at ca. 4000, 3000–2500, and 1000 cal. yr BP, coinciding in timing and duration with well-known dry events in the western Mediterranean and other areas but could have also been generated by local sedimentary or geodynamic processes such as a marine transgression in a subsidence context and extreme wave events (EWEs). The alternation of persistent North Atlantic Oscillation modes probably played an important role in controlling these relatively humid–arid cycles.

Pollen and macrofossil evidence of Late Pleistocene and Holocene treeline fluctuations from an alpine lake in Colorado, USA

High-resolution pollen, plant macrofossil and magnetic susceptibility (MS) data are presented from an alpine lake sediment core from west-central Colorado, recording changes in vegetation and sedimentation for the latest Pleistocene and Holocene (c. the last 12.5 ka; 1 ka = 1000 cal. yr BP). During the Younger Dryas chron (c. 12.9–11.5 ka), Artemisia steppe or tundra grew around the lake, but by the earliest Holocene (10.7–9.5 ka) a subalpine Picea and Abies parkland was established there. Picea remained important through the early Holocene, but also bristlecone and lodgepole pines (Pinus aristata and P. contorta) grew around the lake. Warming conditions are indicated from 9.5 ka, lasting until c. 4.5–3.5 ka, which may have been the warmest period, with greatest development of monsoonal conditions. Trees subsequently retreated downslope from Kite Lake c. 150–200 m during the last 3.5 ka, establishing their present treeline position. A decrease in total Pinus and increases in Artemisia and piñon (P. edulis) indicate a trend toward progressive climate cooling and enhanced winter precipitation. These long-term climatic trends correlate with Holocene changes in summer insolation.

Palaeoenvironments of the Late Miocene Prüedo Basin: implications for the uplift of the Central Pyrenees

The nature, structure and extent of a palaeo-basin sedimentary infill exposed in the Aran valley (Central Pyrenees) was studied by combining stratigraphical and biostratigraphical analyses and an audio-magnetotelluric survey. The basin developed on top of a pre-existing peneplain and was formed by the North Maladeta Fault activity. The fluvio-palustrine sequence filling the basin was at least 100 m thick. Specimens of the taxon Hippuris cf. parvicarpa Nikitin were identified for the first time in a European palaeoflora. The palynological and carpological analyses allowed us to (1) constrain the age of the basin infill as Vallesian (11.1–8.7 Ma), (2) characterize the vegetation of the belt surrounding the basin as a mainly temperate to warm-temperate assemblage, and (3) estimate the palaeoaltitude of the site at between 700 and 1000 m, which leads to an altitude change of 900–1200 m for the North Maladeta Fault downthrown block and 1640–1640 m for the upthrown block. These data allowed us to estimate the regional uplift of the area at between 0.08 and 0.19 mm a−1. The different exhumation values obtained by other researchers for sites located at both sides of the North Maladeta Fault are in agreement with its activity as a normal fault since the Late Miocene. Supplementary materials Details of the data acquisition, processing and modelling for the Porèra profile are available at www.geolsoc.org.uk/SUP18551.

A multiproxy record of postglacial climate variability from a shallowing, 12-m deep sub-alpine bog in the southeastern San Juan Mountains of Colorado, USA

Pollen assemblages, diatom assemblages, and sedimentology, from Cumbres Bog in the southeastern San Juan Mountains of Colorado, provide a record of climate and environmental change since the end of the last glacial maximum (LGM). Cumbres Bog is unusually deep (basal sediments extend 12 m below the surface) for its altitude (~3050 m a.s.l.) and we extracted 7 m core of continuous sediment below ~5 m of water and peat. The resulting record provides strong evidence of: a period of warming immediately after the LGM (~18–13 cal. kyr BP), a cool interval coinciding with the Younger Dryas (~12.8–11.5 cal. kyr BP), a warm stable period from 10 to 6 cal. kyr BP, and a cooler and highly variable climate interval after 6 cal. kyr BP. More specifically, pollen ratios and fossil diatoms indicate that cold periods generally match with previously identified periods of rapid climate change that occurred at 10.6, 8.7–7.9, 7.0–6.9, 5.4–5.2, 3.3–3.0, 2.3, 2.0 and 1.5 cal. kyr BP. This record also adds resolution to previous regional records and indicates that the periodicity of climate variability changed from 2000–3000 years to 700–1100 years around 6 cal. kyr BP and to <500 years after 3.5 cal. kyr BP. Overall, our record provides important, relatively high-resolution paleoclimatic information for this remote region of the southern Rockies.