Modern Analogue Technique Research Articles

Changes in Holocene meridional circulation and poleward Atlantic flow: the Bay of Biscay as a nodal point

Abstract. This paper documents the evolution over the last 10 kyr of one of the key parameters of climate: sea-surface temperatures (SSTs) in the North Atlantic. We focus on the southern Bay of Biscay, a highly sensitive oceanographic area regarding the dynamics of the North Atlantic subpolar and subtropical gyres (SPG and STG respectively). This site furthermore offers unique sedimentary environments characterized by exceptional accumulation rates, enabling the study of Holocene archives at (infra)centennial scales. Our results mainly derive from planktonic foraminiferal association analysis on two cores from the southern Landes Plateau. These associations are used as the basis of modern analogue technique transfer functions to track past hydrographical changes. SST reconstructions were thus obtained at an exceptional resolution and compared to a compilation of Holocene records from the northeastern North Atlantic. From this regional perspective are shown fundamental timing differences between the gyre dynamics, nuancing classical views of a simple meridional overturning cell. Our study highlights that western Europe underwent significant oscillations of (annual) SST during the last 10 kyr. During well-known intervals of mild boreal climate, warm shifts of more than 3 °C per century are accurately concomitant with positive sea-surface temperature anomalies and rise of micropalaeontological indicators of gyre dynamics in the northern North Atlantic, pointing to periods of greater intensity of the North Atlantic Current (SPG cell especially). Conversely, the SST signal records short-term cold anomalies which could be related to weaker SPG dynamics.

Late Quaternary salinity variation in the Lake of Siebleben (Thuringia, Central Germany) – Methods of palaeoenvironmental analysis using Ostracoda and pollen

We test several methods of ostracod-based palaeoenvironmental reconstruction using indicator species approach, mutual ecological/climatic range methods, transfer functions, modern analogue technique and morphological variation within Cyprideis torosa in reconstructing the site evolution of a late Quaternary small lake basin in Thuringia, Central Germany. Sediment sections containing a diverse ostracod fauna were studied and compared with those from modern water bodies of Thuringia. Palynological investigations were executed to reconstruct the environmental conditions in the catchment area and for a obtaining a biostratigraphical framework. The brackish water ostracod Cyprideis torosa as well as the foraminifer Haplophragmoides indicate phases of saline groundwater influence, fed by salt bearing sediments of the Triassic basement. The accompanying freshwater ostracod fauna, however, reflects only low variations of salinity and temperature. Environmental changes in salinity, temperature and ecological stability indicated by microfossils and pollen are caused by an interplay of climatic shifts and the local geological and hydrological setting.

Arctic sea-ice proxies: Comparisons between biogeochemical and micropalaeontological reconstructions in a sediment archive from Arctic Canada

Boxcore 99LSSL-001 from the southwest Canadian Arctic Archipelago (68.095°N, 114.186°W), studied by multiproxy approaches (sea-ice diatom biomarker IP25, phytoplankton-based biomarker brassicasterol, biogenic silica, total organic carbon, dinoflagellate cysts = dinocysts, diatoms) and their applications (sea-ice index PBIP25, modern analogue technique (MAT) transfer functions), provides a chronologically constrained (210Pb, 137Cs, two 14C dates) palaeoenvironmental archive spanning AD 1625–1999 with which to compare and evaluate proxies frequently used in sea-ice reconstructions. Whereas diatoms are rare, PBIP25, biogenic silica and qualitative dinocyst approaches show good agreement, suggesting that palaeo sea-ice histories based on biomarker and microfossil techniques are robust in this region. These combined approaches show fluctuating long open water to marginal ice zone conditions (AD 1625–1740), followed by high-amplitude oscillations between long open water and extended spring/summer sea ice (AD 1740–1870). Greater ice cover (AD 1870–1970) precedes recent reductions in seasonal sea ice (AD 1970–1999). Dinocyst-based MAT, however, produces a low-amplitude signal lacking the nuances of other proxies, with most probable sea-ice reconstructions poorly correlating with biomarker-based histories. Explanations for this disagreement may include limited spatial coverage in the modern dinocyst distribution database for MAT and the broad environmental tolerances of polar dinocysts. Overall, PBIP25 provides the most detailed palaeo sea-ice signal, although its use in a shallow polar archipelago downcore setting poses methodological challenges. This proxy comparison demonstrates the limitations of palaeo sea-ice reconstructions and emphasizes the need for calibration studies tying modern microfossil and biogeochemical proxies to directly measured oceanographic parameters, as a springboard for robust quantitative palaeo studies.

Multi-proxy study of primary production and paleoceanographical conditions in northern Baffin Bay during the last centuries

Abstract As polynyas are often used around the globe as a window on tomorrow's marine polar ecosystems, this study presents reconstructions of primary production and sea-surface conditions near the North Water polynya, since ~ 1560 CE from palynological, isotopic and biomarker analyses of a sediment core. Quantitative reconstructions of sea-surface conditions (temperature, sea-ice cover and production) were derived from the Modern Analogue Technique (MAT) applied to dinoflagellate cyst assemblages. Production was also qualitatively estimated from the stable isotope composition of organic carbon and nitrogen and the concentration of organic biomarkers (IP 25, dinosterols). The results show relatively stable oceanographic conditions but suggest a slight warming accompanied by an increase in productivity after 1860 CE. The comparison of MAT reconstructions and the organic biomarker data suggests that IP 25 provides information about primary production associated with sea-ice but does not unequivocally reflect sea-ice concentration in this regional setting, which is marked by dense seasonal sea ice except during the short summer season.

The Lateglacial interstadial at the southeastern limit of the Sonoran Desert, Mexico: vegetation and climate reconstruction based on pollen sequences from Ciénega San Marcial and comparison with the subrecent record

The last glacial–interglacial transition encompassed rapid climate oscillations that affected both hemispheres. At low latitudes, the pattern of oscillations is not well established. To address this issue, pollen analysis was performed at Ciénega San Marcial, a monsoon‐influenced site located on the southeastern edge of the Sonoran Desert at the limit of the tropical thornscrub. The pollen record covers the Late Wisconsinan glacial terminationII, from 15 650 to 13 400 cal. aBP, includingGS‐2 and the Lateglacial interstadial, and a recent historical period (ADc. 1919 to 2004). We applied the modern analogue technique, in which pollen taxa are assigned to plant functional types (PFTs), to reconstruct the past climates. At the end ofGS‐2, aJuniperus–Pinuswoodland is indicative of annual temperatures 10±2 °C colder than present and higher annual precipitation dominated by winter rains. The onset of the Lateglacial interstadial occurs atc. 15 500 cal. aBP, resulting in a lower sedimentation rate and the spread of a xeric grassland. This period is associated with an increase in summer insolation. A weak signal of summer monsoon intensification is dated to 14 825 cal. aBPbut is associated with colder winter temperatures. A wider spread of tropical taxa occurs after 13 800 cal. aBP, along with the loss ofJuniperus, suggesting a temperature increase of approximately 3 °C. In spite of the earlier Lateglacial warming, the transition from glacial to interstadial conditions seems to be related to North Atlantic atmospheric variations. We conclude that during the last glacial–interglacial transition, the Sonoran Desert at 28.5° latitude was sensitive to climate variations originating in northern latitudes. The recent historical sequence displays summer‐dominant precipitation and additional drivers of climate change, including anthropogenic factors and El Niño, thus showing a stronger Pacific circulation influence in the subrecent period.

USING MULTI-FORAMINIFERAL-PROXIES TO RESOLVE THE PALEOGEOGRAPHIC HISTORY OF A LOWER MIOCENE, SUBDUCTION-RELATED, SEDIMENTARY BASIN (WAITEMATA BASIN, NEW ZEALAND)

A combination of five techniques was used to infer the paleobathymetry and paleoecology of 103 foraminiferal faunas (>63μm, census counts of 384 benthic species) from throughout the lower Miocene Waitemata Basin, Auckland, northern New Zealand. A canonical correspondence analysis ordination showed that environmental factors related to paleo-bathymetry were the main drivers of benthic foraminiferal composition and samples were sorted in approximate paleo-depth order using increasing planktic percentage and species diversity proxies. Cluster analysis recognised seven associations (A–G) and eight subassociations, which were interpreted in terms of paleo-bathymetry and paleoproductivity by qualitative comparison with modern faunas. Planktic-percentage-based regression (with benthic stress-indicator taxa removed) and a Modern Analogue Technique comparison (at generic level) of fossil benthic assemblages with a modern dataset (626 faunas from 0–5000 m) were both used to provide quantitative paleo-bathymetry estimates for each Miocene fauna. These paleo-depth estimates were refined using the upper- and lower-depth limits of some of the less common benthics present. No one technique appears to be better than others. All can provide anomalous paleo-bathymetric estimates that need to be explained. Paleo-depth estimates are most consistent and precise for inner–mid shelf faunas and become less precise and more frequently inconsistent with increasing depth. A number of faunas with inconsistent paleo-bathymetric estimates contain both shallow- and deep-water-restricted benthic tests. These are interpreted as resulting from the inclusion of reworked deep-water fossil tests into shallow faunas, or mixing of contemporaneous tests as sediment flowing from shelf depths down into the basin in turbidity currents and debris flows. The eight bathyal–abyssal associations and subassociations have overlapping depth ranges but only 3–4 of them were present in the basin at any one time. Their geographic distribution reflects a crude depth-related zonation, whereas their stratigraphic succession is inferred to have resulted from a period of increased carbon flux and decreased bottom oxygen sandwiched between periods of lower carbon flux and more oxic bottom conditions. The recognised associations are: A, Elphidium , intertidal–subtidal beach; B, Nonionella-Notorotalia , sheltered inner shelf; C, Amphistegina-Cibicides , exposed inner–outer shelf; D, Astrononion-Gyroidina , ≥mid-bathyal; E, Bolivina , mid–lower bathyal; F, Cibicides-Bolivina , bathyal; G, Oridorsalis-Neugeborina-Pullenia , ≥lower bathyal. The inferred paleoenvironment of the faunas is used to help resolve the paleogeographic history of the Waitemata Basin. Thin basal transgressive sequences of lensing conglomerate, shelly sandstone and limestone containing shelf faunas (Assocs A–C) record initiation of basin subsidence over a wide area (300 × 100 km), ~21 Ma. Continued subsidence (estimated from foraminiferal data at ~1 mm/yr) was accompanied by sediment starvation with little or no sediment accumulating until the basin reached lower bathyal depths (F, G). This created sufficient slope for turbidity currents from the north to flow down submarine canyons and deposit three interfingering submarine fans of sandstone (800–1000 m thick). Displaced Cretaceous–Oligocene nappes and melange (Northland Allochthon) moved part way into the basin from the north and large sub-seafloor slides of mixed basinal sediment and allochthon slid off its advancing toe. These slides were triggered by uplift of the northern part of the basin, which created land fringed by a wide shelf (C). A large submarine stratovolcano built up on the west side of the basin and shed volcaniclastic sediment (C, D, F) down its eastern slopes with some debris flows transporting mixed shallow water faunas up to 30 km into the basin. The ~4 million-year-life of this subduction-related basin came to an end with complete eversion before the end of the early Miocene.

Improving past sea surface temperature reconstructions from the Southern Hemisphere oceans using planktonic foraminiferal census data

AbstractWe present an improved database of planktonic foraminiferal census counts from the Southern Hemisphere oceans (SHO) from 15°S to 64°S. The SHO database combines three existing databases. Using this SHO database, we investigated dissolution biases that might affect faunal census counts. We suggest a depth/ threshold of ~3800 m/ = ~ −10 to −5 µmol/kg for the Pacific and Indian Oceans and ~4000 m/ = ~0 to 10 µmol/kg for the Atlantic Ocean, under which core‐top assemblages can be affected by dissolution and are less reliable for paleo‐sea surface temperature (SST) reconstructions. We removed all core tops beyond these thresholds from the SHO database. This database has 598 core tops and is able to reconstruct past SST variations from 2° to 25.5°C, with a root mean square error of 1.00°C, for annual temperatures. To inspect how dissolution affects SST reconstruction quality, we tested the data base with two “leave‐one‐out” tests, with and without the deep core tops. We used this database to reconstruct summer SST (SSST) over the last 20 ka, using the Modern Analog Technique method, on the Southeast Pacific core MD07‐3100. This was compared to the SSST reconstructed using the three databases used to compile the SHO database, thus showing that the reconstruction using the SHO database is more reliable, as its dissimilarity values are the lowest. The most important aspect here is the importance of a bias‐free, geographic‐rich database. We leave this data set open‐ended to future additions; the new core tops must be carefully selected, with their chronological frameworks, and evidence of dissolution assessed.

Impact of eustatic and tectonic processes on the southeastern Mediterranean shelf during the last one million years: Quantitative reconstructions using a foraminiferal transfer function

Benthic foraminifera-based transfer function models provide accurate sea-level reconstructions for shelf and coastal environments around the world. This study presents for the first time results of a foraminiferal transfer function aiming at the reconstruction of relative sea-level in the southeastern Mediterranean shelf during the past one million years. A Weighted Averaging-Partial Least Squares (WA-PLS) regression method was used for the development of the transfer function based on an extensive set of surface samples (modern training data set) from the study area. The transfer function was applied to three shelf sediment successions retrieved from Haifa Bay, Israel. The Modern Analog Technique (MAT) and the random TF test were used to investigate similarity between modern and fossil foraminiferal faunas, testing the reliability and significance of our model for paleowater depth reconstruction. The performance of the WA-PLS transfer functions suggests a strong relationship between the observed and estimated water depths in the modern data set with relatively precise reconstructions of 7.0m (Root Mean Squared error of Prediction) over a water depth range of 3 to 80m. The deepest paleo water depths were reconstructed for the earliest sea-level highstands recorded in the sediment records (MIS 27? and MIS 29?) in the down-faulted boreholes due to tectonic movements, whereas the younger interglacial sedimentary units (MIS 25?, 13, 11, 7, 5.5 and 1) were mostly deposited at <15m water depths or slightly deeper during MIS 7. The relative sea-level history of the southeastern Mediterranean shelf reveals deviations from the global sea-level trend during MIS 5.5, which may have resulted from minor tectonic movements in the study area and/or un-representation of the phase of maximum sea-level in the sediment succession. Our paleowater depth estimates demonstrate a shallowing trend during the Holocene, generally matching other regional Mediterranean sea-level records.

Orbital obliquity cycles recorded in Kuroshio Current region, eastern Asia, around Plio–Pleistocene boundary

Global climate underwent a period of significant cooling at the Plio–Pleistocene Transition (∼2.6 Ma). The influence of this change on the Kuroshio Current region in the Pacific Ocean, off eastern Asia, is not well known. In this study, we clarify temporal changes in the paleoenvironment under the influence of the Kuroshio Current during the late Pliocene and early Pleistocene using high-resolution faunal proxy records of fossil Ostracoda (Crustacea). The study unit is the Ananai Formation in the southeastern region of Shikoku, southwest Japan. The modern analog technique (MAT) is employed for the quantitative estimation of paleo-bottom water temperatures (PBWTs) and paleo-water depth (PWD) during the deposition of the formation. Ostracode MAT results show PBWT fluctuations during warmest and coldest months, with values of 16°C–20 °C and 12°C–16 °C, respectively, and a PWD of 70–140 m, reflecting sea-level oscillations. Moreover, the PBWT in the coldest month is 3 °C–4 °C lower than present-day water temperatures at the same shallow water depths. Temporal changes in these paleoenvironmental variables based on MAT are in good agreement with global oxygen isotope records. Orbital obliquity cycles with 41-kyr periodicity are recorded for the first time in an onshore section in the Kuroshio Current region at the Plio–Pleistocene boundary interval.

Stream diatom assemblages as predictors of climate

Summary Benthic diatoms have been widely used as indicators of water quality in streams. New insights that diatoms may also respond to large‐scale drivers, such as climate or historical factors, highlight the need to reassess the usefulness and the reliability of diatoms as bioindicators. Using a suite of modelling techniques, weighted averaging (WA), weighted averaging partial least squares, modern‐analogue technique (MAT) and two machine learning techniques, boosted regression trees (BRT) and random forests (RF), we calibrated models to infer water quality and climatic variables using diatom abundance data collected from 227 stream sites in Finland. Predictive ability was generally better for climatic variables [growing degree days (GDD) defined as temperature &gt;5 °C, summer precipitation and water balance] than for local environmental variables (conductivity, water colour and total phosphorus). The strongest relationships were found for GDD (r2 = 0.86, MAT) and conductivity (r2 = 0.82, RF). Using BRT, we also identified potential indicator species for local environmental and climatic variables, based on relative importance species in the models. Our results show that diatoms could serve as efficient proxies for climatic variables and local environmental conditions. Furthermore, new modelling techniques such as modern regression trees can provide new insights into relationships between diatom assemblages and local water quality and climate, and thus help to construct more reliable indices. These methods could also serve as important tools to infer environmental variables in changing ecosystems.

Dinoflagellate cyst based modern analogue technique at test — A 300 year record from the Gulf of Taranto (Eastern Mediterranean)

To test the performance of dinoflagellate cyst based palaeoenvironmental reconstructions derived via the modern analogue technique (MAT) and a non-quantitative method, we have compared, reconstructions of mixed layer temperature, sea surface temperature and mixed layer salinity of a well dated site from the Gulf of Taranto, to instrumental data of air temperature, precipitation and river discharge. Two hypotheses are tested: the reconstructions of dinoflagellate cyst based MAT improve when a) a local reference dataset rather than an extra-regional reference dataset is used, b) the reference and downcore datasets are corrected for species specific preservation prior to analysis. This is achieved by executing four experiments based on different reference datasets 1) a North Atlantic dataset including all species, 2) a North Atlantic dataset including degradation resistant species, 3) a Mediterranean dataset including all species, 4) a Mediterranean dataset including degradation resistant species only.We show that MAT based reconstructions improve when a local rather than an extra-regional reference dataset is used. Exclusively including species resistant to degradation improves the reconstruction if an extra-regional reference dataset is used, but leads to considerable loss of variability if the local reference dataset is used.Both MAT-based and qualitative reconstructions correlate to instrumental data for industrial times. For pre-industrial times, MAT reconstructions of variability in temperature and salinity do not co-vary with variability in the instrumental data, whereas qualitative reconstructions provide a relatively good fit. We suggest this to be a result of the anthropogenic influence on coastal marine ecosystems existing today at the majority of the coastal sample sites in the reference dataset. As a result, conditions at these sites do not correspond to pre-industrial conditions. We advise users of dinoflagellate cyst transfer functions to pay more attention to this aspect.

Quantitative paleoclimate reconstruction based on a 130 ka pollen record from the C9001C core off NE Japan

Although monsoons are an important climatic factor in the Indo-Pacific region, the mechanism responsible for monsoon variations is not fully understood, particularly at the 105 year-scale, which includes the Milankovitch orbital cycles. Here, we provide a palynological result from the C9001C marine core drilled in the NW Pacific off the Shimokita Peninsula, eastern Japan. Based on the parallel δ18O record, the pollen record covers the past 130,000 years. During the interglacial periods (MIS1 and 5e), the C9001C core is dominated by cool temperate pollen of Fagus (beech) and Quercus subgenus Lepidobalanus (deciduous oak) that is similar to the present-day Shimokita vegetation. However, during the glacial periods (MIS2-4), the pollen spectra are dominated by subarctic elements of Betula (birch) and Picea (spruce), similar to the vegetation of Sakhalin and northern Hokkaido. These fossil pollen data are converted into quantitative climate variables by using the modern analog technique. Of the reconstructed paleoclimate variables, the mean annual temperature (Tann) correlates with the parallel δ18O record. By contrast, reconstructed precipitation in the summer (Psum) shows a similar pattern to the northern insolation variations particularly during MIS5, though the phase relation between the two curves requires further studies.

Quantitative woody cover reconstructions from eastern continental Asia of the last 22 kyr reveal strong regional peculiarities

We present a calibration-set based on modern pollen and satellite-based Advanced Very High Resolution Radiometer (AVHRR) observations of woody cover (including needleleaved, broadleaved and total tree cover) in eastern continental Asia, which shows good performance under cross-validation with the modern analogue technique (all the coefficients of determination between observed and predicted values are greater than 0.65). The calibration-set is used to reconstruct woody cover from a taxonomically harmonized and temporally standardized fossil pollen dataset (including 274 cores) with 500-year resolution over the last 22 kyr. The spatial range of forest has not noticeably changed in eastern continental Asia during the last 22 kyr, although woody cover has, especially at the margin of the eastern Tibetan Plateau and in the forest-steppe transition area of north-central China. Vegetation was sparse during the LGM in the present forested regions, but woody cover increased markedly at the beginning of the Bølling/Allerød period (B/A; ca. 14.5 ka bp) and again at the beginning of the Holocene (ca. 11.5 ka bp), and is related to the enhanced strength of the East Asian Summer Monsoon. Forest flourished in the mid-Holocene (ca. 8 ka bp) possibly due to favourable climatic conditions. In contrast, cover was stable in southern China (high cover) and arid central Asia (very low cover) throughout the investigated period. Forest cover increased in the north-eastern part of China during the Holocene. Comparisons of these regional pollen-based results with simulated forest cover from runs of a global climate model (for 9, 6 and 0 ka bp (ECHAM5/JSBACH ∼1.125° spatial resolution)) reveal many similarities in temporal change. The Holocene woody cover history of eastern continental Asia is different from that of other regions, likely controlled by different climatic variables, i.e. moisture in eastern continental Asia; temperature in northern Eurasia and North America.

Salt-marsh foraminiferal record of 10 large Holocene (last 7500 yr) earthquakes on a subducting plate margin, Hawkes Bay, New Zealand

Sudden changes in microfossils and lithologies in Holocene sediments of a former tidal inlet on the Hikurangi subduction margin provide evidence of 10 large earthquakes. Studies were focused in three former embayments where intertidal shelly sediment interfingers with freshwater and salt-marsh peat. Paleoelevation histories were reconstructed using the modern analogue technique with foraminiferal assemblages. Land elevation record analysis indicates 8−9 m of mid- to late Holocene tectonic subsidence occurred prior to 1.5 m of uplift during the A.D. 1931 Hawkes Bay earthquake. Chronologies of displacement events were constrained using 50 radiocarbon dates and three widespread air-fall tephras. We infer the following earthquakes: earthquake 1: 7.3−7.0 ka (−1.1 ± 0.3 m), earthquake 2: 5.6−5.1 ka (+0.4 ± 0.4 m), earthquake 3: 5.2−4.9 ka (−0.5 ± 0.5 m), earthquake 4: 4.4−3.8 ka (−0.6 ± 0.5 m), earthquake 5: 2.8−2.4 ka (−0.9 ± 0.5 m), earthquake 6: 1.73−1.70 ka (−1.0 ± 0.3 m), earthquake 7: 1.5−1.3 ka (−0.7 ± 0.5 m), earthquake 8: 1.04−0.89 ka (−1.2 ± 0.4 m), earthquake 9: 0.60−0.44 ka (−0.8 ± 0.6 m), and earthquake 10: A.D. 1931 (+1.5 ± 0.3 m). A further 1.6−2.6 m of subsidence could have occurred by gradual aseismic slip or in smaller earthquakes. The age ranges of four of the recognized earthquakes (earthquakes 1, 6, 8, and 9) overlap with other documented displacement events onshore along 250−600 km of the Hikurangi subduction margin, and with turbidites offshore 100−300 km to the north. These four are considered strong candidates for large subduction-interface earthquakes. The other five inferred earthquakes are less strongly correlated with along-margin displacement events and offshore turbidites. These could have been caused by upper-plate fault ruptures (like historic earthquake 10), but subduction-interface sources cannot be ruled out. This evidence for repeated coseismic vertical deformation suggests large coseismic slip on a part of the subduction interface beneath Hawkes Bay that is currently dominated by aseismic creep processes, such as transient slow-slip events. This clearly indicates multiple slip processes are possible in a single location on a subduction interface.

Development of coccolithophore-based transfer functions in the western Mediterranean sea: a sea surface salinity reconstruction for the last 15.5 kyr

Abstract. A new data set of 88 marine surface sediment samples and related oceanic environmental variables (temperature, salinity, chlorophyll a, oxygen, etc.) was studied to quantify the relationship between assemblages of coccolithophore species and modern environmental conditions in the western Mediterranean Sea and the Atlantic Ocean, west of the Strait of Gibraltar. Multivariate statistical analyses revealed that coccolithophore species were primarily related to sea surface salinity (SSS), explaining an independent and significant proportion of variance in the coccolithophore data. A quantitative coccolithophore-based transfer function to estimate SSS was developed using the modern analog technique (MAT) and weighted-averaging partial least square regression (WA-PLS). The bootstrapped regression coefficient (R2boot) was 0.85MAT and 0.80WA-PLS, with a root-mean-square error of prediction (RMSEP) of 0.29MAT and 0.30WA-PLS (psu). The resulting transfer function was applied to fossil coccolithophore assemblages in the highly resolved (~ 65 years) sediment core CEUTA10PC08 from the Alboran Sea (western Mediterranean) in order to reconstruct SSS for the last 25 kyr. The reliability of the reconstruction was evaluated by assessing the degree of similarity between fossil and modern coccolithophore assemblages and by a comparison of reconstructions with fossil ordination scores. Analogs were poor for the stadials associated with Heinrich events 2 and 1 and part of the Last Glacial Maximum. Good analogs indicate a more reliable reconstruction of the SSS for the last 15.5 kyr. During this period, several millennial and centennial SSS changes were observed and associated with sea-level oscillations and variations in the Atlantic Water entering the Alboran.

Approaches to quantitative reconstruction of woody vegetation in managed woodlands from pollen records

There has been increasing interest in developing quantitative methods for reconstructing the dynamics of cultural landscapes over the last 15years. This paper adds to this literature by using various approaches to reconstruct the vegetation of two woodlands subject to rotational coppicing (the periodic cutting of broadleaved trees and shrubs for wood products). Pollen deposition at ground level was determined at both sites using ‘Tauber’ traps placed near to the centre of 14 compartments of differing age in the coppice rotation. For the main woody taxa, Relative Pollen Productivity (RPP) estimates were derived using linear regression for pollen influx data and Extended R-value analysis for percentage data. The vegetation around three ponds was reconstructed by applying four methods (inverting the two RPP estimate approaches, the modern analogue technique and correction for pollen productivity using the linear regression estimated RPP values) to pollen data obtained from the uppermost sample of sediment from the ponds. To determine whether these methods gave better estimates of the vegetation composition than the original pollen proportions, the results were compared with the surveyed vegetation around each pond using the Bray–Curtis Index. Linear regression of pollen influx produced RPP values which are comparable with previous European studies, whilst for some taxa the Extended R-value analysis produced estimates which are orders of magnitude different both from values derived from the linear regression and previous work. No single approach performed equally well at reconstructing the vegetation around the ponds, and at two of the three locations the uncorrected pollen proportions were most similar to the surveyed vegetation. We conclude that applying quantitative reconstruction methods to individual small sites is, currently, not likely to be useful in complex cultural landscapes. In the context of coppiced woodland, deficiencies in our understanding of pollen taphonomy and the impact of the practice on pollen production first need to be rectified, and we identify strategies to address this situation.

Variability of the upwelling system in the southeastern Brazilian margin for the last 110,000 years

Western boundary upwelling systems such as those found in the southeastern Brazilian margin (SBM) are promoted by special atmospheric and oceanographic interactions with topography of the coastline. In order to investigate the evolution of the upwelling system in SBM over the past 110kyr, the abundance of Globigerina bulloides (a planktonic foraminiferal species typically associated with upwelling waters) from two marine cores was combined with estimates of upper water thermal structure derived from Modern Analog Technique based on planktonic foraminifera. High abundances (up to 35%) of large specimens of G. bulloides (≥250μm) associated with cooling subsurface waters reveal the occurrence of an intense coastal upwelling system between 110kyr and 88kyr. Changes in coastline orientation due to lower sea level stand (20–70m lower than today) and a longer season of vigorous Brazil Current (BC) and prevailing northeasterly winds maintained permanently the cold and nutrient-rich South Atlantic Central Waters (SACW) over the shelf promoting the strongest upwelling system of the Brazilian coast over the past 110kyr. At 88kyr, a sharp reduction of 14% in abundance of G. bulloides and a 2.6°C warming of the water column indicates an abrupt weakening of the coastal upwelling, probably caused by a disturbance in SACW formation. During the last glaciation, longer current winter-like conditions of prevailing southwesterly winds and a weakened Brazil Current suppressed the upwelling system in SBM. At this time, constant and relatively high abundance of G. bulloides (up to 10%) suggest that the coastal upwelling was restricted to a short period of months year-round, when the BC and northeasterly winds were relatively vigorous despite the glacial context. The predominance of warm and oligotrophic Tropical Waters due to the shutdown of coastal upwelling in northern SBM was inferred for the last 20kyr. Despite vigorous northeasterly winds and BC strength, high sea level stand during the Holocene submersed the Abrolhos Bank (AB) leading to a new coastline configuration. Presently, the interaction between the AB and the BC generated a new oceanographic feature in SBM, the Vitória Eddy, with very limited efficiency in producing a strong upwelling such as the one recorded in the past.

TAPHONOMICALLY- AND INFAUNALLY-ADJUSTED SALT MARSH FORAMINIFERAL RECORD OF LATE HOLOCENE EARTHQUAKE DISPLACEMENTS AND A TSUNAMI SAND, NEW ZEALAND

Most paleoenvironmental assessments of fossil salt-marsh foraminiferal faunas are based on modern analogue samples from the surface 1–2 cm. Usually no account is taken of the faunal modifications that result from the 60–90% of the fauna that live at greater infaunal depths or from the patchy loss of agglutinated tests that mostly occurs in the oxic and taphonomically active zone (TAZ, upper 10–15 cm). Here we provide examples of the highly variable foraminiferal test distribution in New Zealand salt marsh cores and surface transects. Using these, we suggest two simple adjustments that could be made to quantitative estimates of paleoelevation derived from fossil salt marsh faunas based on modern surface analogues: 1) Determine a minimum dead specimen density (8 per cm3 in this study) below which all fossil faunas are considered to have suffered significant taphonomic loss (in the TAZ) and their paleoelevation estimates are deemed suspect and may be applied tentatively to core depths ~15+ cm above. 2) Move the core depths of all acceptable paleoelevation estimates upwards by the mean infaunal depth of the dominant species (2.5–8 cm in this study). We applied these adjustments to two late Holocene cores located 5 m apart in salt marsh in tidal Big Lagoon, Marlborough, New Zealand. Fifty percent (18) of the fossil faunas have unacceptably low specimen densities (<8 per cm3). A 15 cm thick fine pebbly sand bed contains a mix of calcareous tests derived from subtidally offshore and from intertidally within the adjacent sheltered lagoon, and agglutinated tests of salt marsh taxa inferred to be displaced and/or infaunally in-situ. These faunas provide strong evidence for sand deposition on top of the high salt marsh by the outgoing surge of a tsunami. The upper 7 and 13 cm of the sand lacks calcareous tests, which are inferred to have been dissolved by acidic pore waters following colonisation by salt marsh vegetation. Our taphonomically- and infaunally-adjusted paleoelevation estimates, generated by modern analogue technique using a training set of 1017 modern New Zealand faunas, provide near-identical elevational histories for both cores. Both core sequences (70 and 85 cm thick) were deposited within a 50 cm-wide elevational envelope in middle- and high-tidal salt marsh. Both record an inferred 0.5 m co-seismic subsidence event coincident with emplacement of the tsunami sand (~840 cal yrs BP) and a second smaller, ~0.3 m, presumably co-seismic, subsidence event coincident with a sharp peat-mud contact in one core (~500 cal yrs BP).

Foraminiferal record of the 2010–2011 Canterbury earthquake sequence, New Zealand, and possible predecessors

Abstract This study documents changes in elevation, plant zones and foraminiferal faunas along a transect through salt marsh taken 14 months prior to, and 12 and 33 months after the Feb 2011 Canterbury Earthquake, which devastated the city of Christchurch, New Zealand. The saltmarsh subsided 0.1 m relative to the adjacent sand spit due to shaking-induced lateral spread. Over the 33-month period the lower 0.3 m elevation of the rush zone died away and a pine tree plantation on the sand spit terrace (~ 0.3–0.5 m above highest astronomical tide, HAT in 2009) also died and was removed. The foraminiferal faunal composition changed only slowly following the earthquake, except in the highest stations. Here their landward extent had migrated 0.45 m upwards by the end of 2013 and the foraminifera-based elevation estimates (Modern Analogue Technique, MAT) indicated a total subsidence of 0.36–0.62 m, similar to that indicated by LiDAR surveys. Elevation estimates based on the dead foraminiferal faunas in replicate salt-marsh cores 10 m apart indicate a subsidence event of ~ 0.15–0.2 m at 45 cm downcore. This was probably a result of shaking-induced lateral spread of the marsh into the river channel during a late 19th century earthquake—most probably the 1869 Christchurch Earthquake. A second possible co-seismic subsidence event is indicated at 80 cm downcore but the MAT elevational estimates are less reliable because of the low density foraminiferal faunas. A sudden relative fall in sea-level (or uplift of land) of 0.2–0.25 m is indicated by MAT elevation estimates at 60 cm downcore. As there is no known supporting evidence for a tectonic uplift event, we infer the most likely explanation is that this event spans an erosional time break (> 500 yrs). Although we did not recognise its significance at the time, the foraminiferal record in our 2009 salt marsh core indicated that Christchurch had previously experienced significant co-seismic shaking on at least one, and maybe more, occasions within the last 1000 yrs.

Diachronous evolution of sea surface conditions in the Labrador Sea and Baffin Bay since the last deglaciation

Assessing changes in sea surface conditions due to the effects of past freshwater outflow through Baffin Bay and Davis Strait to the Labrador Sea, hereafter referred to as the Baffin Bay corridor, is relevant in understanding the variability in Labrador Sea Water (LSW) formation. Here, regional changes in oceanographic circulation and sea surface conditions are reconstructed based on organic-walled dinoflagellate cyst (dinocyst) assemblages from four cores collected from deep, central sites of the Baffin Bay corridor. All cores exhibit a major shift in dinocyst assemblages since the late glacial period. This shift consists of a change from a polar–subpolar heterotrophic species assemblage tolerating cold and near permanent ice-covered conditions, to assemblages characterized by a higher diversity and the occurrence of phototrophic taxa associated with mild conditions. Sea surface reconstructions from the modern analogue technique display a shift from harsh, quasi-perennial ice cover to warmer summer sea surface temperatures and a seasonal sea ice. South of the Davis Strait sill, this regime shift occurred at ca. 11.9 cal ka BP due to the influence of North Atlantic waters. Baffin Bay, however, remained densely sea ice covered until about 7.4 cal ka BP, when these warmer waters penetrated into Baffin Bay and mixed with the West Greenland Current (WGC). This mixing was facilitated by the retreat of the Greenland and Laurentide Ice Sheet (LIS) margins. A major change in Labrador Sea surface conditions occurred nearly at about the same time (~7.6 cal ka BP) when the strong stratification of surface waters weakened because of the reduction in meltwater supplies from the LIS that allowed winter convection and the inception of LSW formation. All these new records demonstrate large amplitude fluctuations in sea surface conditions tightly controlled by the relative strengths and shifts of the warmer WGC and colder Baffin Island Current.