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Cross-comparison of last glacial radiocarbon and OSL ages using periglacial fan deposits
(Elsevier, 2020-10-08)
Two cores from a Weichselian periglacial alluvial fan were dated using 14C and OSL, to verify the reliability of both methods and check the upper dating limit of the 14C method. Both dating methods yielded a similar chronology for core Eerbeek-I, with infinite 14C dates for the lower part where OSL dates indicated ages of over 45 ka. Finite 14C dates were obtained throughout the core for Eerbeek-II, despite stratigraphic and OSL evidence suggesting ages beyond 14C limits. Apparently, additional chemical pre-treatment to remove younger carbon fractions did not work adequately for samples from this core. We hypothesize that this may be related to a larger influence of younger-age humin fractions in the mainly sandy Eerbeek-II deposits compared to those buffered by a thick peat layer of Eerbeek-I. We suggest that (local) stratigraphy, percolation and humification processes may impact 14C ages of organic deposits more than commonly assumed, and should receive more attention. In addition, we introduce a new method to assess robustness and validity of OSL dates and demonstrate the applicability of OSL dating methods in this setting. Our results highlight that the 14C method requires additional verification methods, such as OSL, for deposits older than 30 ka.
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Forum communication comment on “A multidisciplinary overview of the lower Miño River terrace system (NW Iberian Peninsula)” by E. Méndez-Quintas et al
(Elsevier, 2020-11-10)
The paper by E. Méndez-Quintas et al. (2020) aims to give an interdisciplinary overview of the lower Miño River terrace record in NW Iberia by combining geological and archaeological data. The authors also pretend to re-interpret the geological-geomorphological evolution of this river by presenting new data and by comparing them to data published in our five related papers. However, a wrong interpretation of the data they present as well as incorrect and selective citations of ours lead to the production of a publication containing numerous errors. The most flagrant ones are the mapping of fluvial terraces on one side of the river only and an incorrect interpretation of the sedimentological information used to correlate terrace remnants along-river. This results in incorrect longitudinal profile reconstructions and an improper proposal of the evolution of the Miño terraces that is even contradicted by the data they present. On the upside, the ages of the Porto Maior terrace site published by these authors confirm our previously published longitudinal profile reconstructions and evolutionary model of the Miño valley.
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First record of OSL‐dated fluvial sands in a tropical Andean cave reveals rapid late Quaternary tectonic uplift
(Wiley, 2020-12-05)
Abstract We present the first results of OSL‐dated fluvial sands extracted from a riverside cave in the tropical Andes. The excellent agreement between the ages of the various samples allowed the calculation of a late Quaternary valley incision rate forced by ongoing uplift of an active Subandean fault‐propagation fold in NE Peru. A structural cross‐section was constructed to understand the relationship between the geometry of the fault‐propagation fold, historical damaging earthquakes and the cave system. The calculated uplift rate is 2.3 to 2.6 mm a−1 over the past 70 ka and can be directly linked to active propagation of west‐verging basement thrusts. It is similar to uplift rates calculated from fluvial terraces in the Subandes of Colombia and Venezuela. The results will help to better assess the seismic hazard and confirm that OSL dating of fluvial sands in caves is a powerful tool to quantify uplift rates of active mountain fronts.
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Fluvial terraces as quantitative markers of late Quaternary detachment folding and creeping thrust faulting in the Peruvian Huallaga basin
(Elsevier, 2020-10-15)
In the western Huallaga basin, located in the Peruvian Subandes, a series of nine fluvial terraces with elevations of up to 160 m above current floodplain level (+FP) were formed against the Biabo anticline. The terraces are progressively more folded with age and terrace elevation and present growth strata, suggesting ongoing folding of the anticline. Optically and InfraRed Stimulated Luminescence (OSL and IRSL) dating of fluvial terrace sediments yielded ages of 10.8 ± 0.7 ka for the T2, 24.2 ± 2 ka for the T4 and 46 ± 9 ka for the T7 terrace. Uniform uplift rates of 2 mm a−1 for the past 46 ka were calculated. Terrace profiles and two new seismic profiles that cross the Biabo anticline longitudinally and transversally were used to calculate shortening rates of 2.6 mm a−1 for the anticline over the past 46 ka. Our calculations took into account potential contributions of detachment thrusting parallel to the Biabo fold and reverse tear faulting perpendicular to the fold tip as visible on the seismic sections. A maximum uplift rate of 0.62 mm a−1 due to reverse thrusting of the Bellavista tear fault for the past 46 ka could have contributed to the overall uplift of the Bellavista terraces. Based on trigonometric relationships, a reverse fault slip rate of 0.86 mm a−1 and an age of 137 ka were calculated for this fault. In the eastern part of the Huallaga basin, a series of two fluvial terraces were formed with elevations of 13–15 m and 30 m + FP and with OSL ages of 6.9 ± 0.7 ka for T1 and 30.2 ± 3.1 ka for T2. The interpretation of a third new seismic profile crossing the study area in combination with field observations and trigonometric relationships shows that the terraces were formed by a complex system of oblique ramp and strike-slip faulting resulting in uplift and shortening rates of 1 mm a−1 and 1.96 mm a−1 between 30.2 ka and 6.9 ka respectively; and uplift and shortening rates of 2 mm a−1 and 3.70 mm a−1 from 6.9 ka until the present. Reverse slip of the Shapaja oblique ramp, that was responsible for uplift of the terraces, was calculated at 2.19 mm a−1 for the period 30.2 to 6.9 ka and 4.15 mm a−1 from 6.9 ka until the present. Measured total slip in combination with the calculated slip rates allowed an age calculation of 181 ka for the Shapaja thrust ramp. Our inferences showed that the studied tectonic structures absorb most of the present-day deformation in the Huallaga basin. The calculated uplift and shortening rates are compatible with published, present-day crustal movement velocities, and Miocene to present shortening rates for various, studied sites of the tropical Subandes and suggest that it is deforming at a relatively uniform rate. The timing of sedimentation of the terrace sediments may have corresponded to periods of increased precipitation which occurred coeval with the Heinrich events of the Northern Hemisphere. Terrace records similar as ours have been described for other, tectonically highly active areas in the tropical Subandes, but not in the tectonically less active Amazonian lowlands. It is suggested that uplift rates of at least 1 mm a−1 are needed to preserve suborbital climate cycles as individual terraces. The tropical Subandes could therefore be a new key area to investigate the interactions between tectonics, suborbital climate forcing and fluvial response during the late Quaternary.
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The earliest adobe monumental architecture in the Americas
(National Academy of Sciences, 2021-11-30)
Adobe bricks, or mud bricks, are construction elements which have defined major architectural traditions in the Andes over thousands of years. From Moche pyramids and the ancient city of Chan Chan in pre-Hispanic times, to Spanish casonas of the colonial period and rural houses in contemporary South America, adobe has been a central component in Andean architecture. Discovery of the remains of an early monumental building constructed primarily of adobes at Los Morteros (lower Chao Valley, north coast of Peru) places the invention of adobe architecture before 5,100 calendar years B.P. The unique composition, internal structure, and chronology of the adobes from Los Morteros show the beginnings of this architectural technique, which is associated with El Niño rainfall and the construction of the earliest adobe monumental building in the Americas. We propose that adobe architecture became a major Andean tradition after a long period of technical evolution and experimentation with both shape and composition.
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Assessing the accuracy of combined DEM-based lineament mapping and the normalised SL-index as a tool for active fault mapping
(Elsevier, 2021-08-20)
Satellite image-based lineament mapping in combination with the normalised Stream Length Gradient Index (SL/K-index) is one of the most widely used methods to identify active faults in areas where other verification methods, such as faulted strata, geophysical subsurface data, or seismic activity in the upper crust, are unavailable, and to classify their degree of activity. The accuracy of this approach, however, has not yet been assessed rigorously. We assess its accuracy by comparing the results against 27 thrust fault segments that were identified from six seismic reflection profiles with a total length of 340 km. Over 106,000 SL/K points were calculated from 815 river profiles on basis of the SRTM v.3.0 DEM. Standard deviations (SD) of < -2SD (class 1) to > +2SD (class 5) were calculated from the lognormalised SL/K data and five SL/K classes were made and assigned to the nearby lineaments. A comparison with the fault segments from the seismic profiles showed a good coincidence between the presence of knickpoints, SL/K classes 4 and 5 and to a lesser extent class 3. Lineaments with SL/K classes 1 and 2 never corresponded to actual faults. We identified seven master thrusts of which three are very active to highly active, likely older, structures with important amounts of accumulated slip. The other thrusts are probably younger faults with relatively little accumulated slip, yet they are also active to highly active faults. Minor strike-slip faults were identified as very active to highly active structures. Published slip rates shows that at least some of the SL/K class 4 faults experience slip rates of ~1 to ~4 mm a−1 and the SL/K class 5 strike-slip faults may experience slip rates as high as one metre over the course of several months. Most active faults occur where the basal evaporite layer is abnormally thick.
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An assessment of competing factors for fluvial incision: an example of the late Quaternary exorheic Moyobamba basin, Peruvian Subandes
(Elsevier, 2021-05-01)
Three competing factors for late Quaternary fluvial incision in the Moyobamba piggyback basin, located in the northeastern Peruvian Subandes, were investigated, namely base-level change as a result of basin opening, changes in the ratio sediment load:discharge (Qs: Qw) controlled by orbital and suborbital climate cyclicity, and thrust-related tectonic uplift. To assess the relative importance of these factors, the fluvial terrace staircases of five rivers were studied in terms of their sedimentology, stratigraphy and geomorphology. A new dataset of eight Optically Stimulated Luminescence (OSL) and four 14C ages shows that the final phase of endorheic alluvial fan aggradation lasted until 18.0 ± 1.4 ka, after which basin-wide incision occurred. Incision at the basin outlet as a result of adaptation to the new base level is decoupled from the observed incision in the middle and upper parts of the basin. There, incision is the result of a northwestward increase in uplift related to movements of the Cerro Blanco thrust at the western border of the Moyobamba basin. Incision was continuous from 18 ± 1.4 ka until 10.3 ± 0.7 ka without terrace formation, but between 10.3 ± 0.7 ka and 5.5 ± 0.4 ka up to four terrace levels were formed. Deposition of fluvial terrace sediments may have occurred during short periods characterised by a more intense South American Summer Monsoon (SASM) controlled by suborbital climate cyclicity, but a more precise age control is needed to confirm this theory. Precession-controlled, increased Holocene aridification between 10.3 ± 0.7 ka and 5.5 ± 0.4 ka may have resulted in a decrease in the ratio Qs: Qw leading to a five-fold increase in fluvial incision rates of up to 6.0 mm a−1 and flattening of terrace profile gradients with rates of up to 2.6 mm a−1. After 5.5 ± 0.4 ka, rates dropped again. Over longer timescales, movement of the Cerro Blanco thrust caused basin-wide uplift and fluvial incision at a more constant pace. During the past ~20 ka, incision increases from the southwest basin border towards the northwestern basin border with corresponding uplift rates increasing from 0.2−0.2+0.3 mm a−1 to 2.1−0.2+0.2 mm a−1; and with shortening rates increasing northwestward from 0.2−0.2+0.4 mm a−1 to 2.8−0.2+0.3 mm a−1. The highest, presented uplift rates are in agreement with independently published data for our study area. Our data shows that over longer timescales of ~20 ka, incision rates can be used as proxies for tectonic uplift, but over shorter timescales incision rates may be seriously affected by climate change and cannot be used to infer uplift rates. It is further suggested that continuous uplift and erosion of the Cerro Blanco thrust system, and subsequent sedimentation in the basin, in combination with increased precipitation during Heinrich events 1 and 2 and the Last Glacial Maximum (LGM), led to basin overflow. Simultaneously, river capture through headward erosion, guided by strike-slip faulting, may have led to the definitive opening up of the Moyobamba basin. To our knowledge, this is the first time that late Quaternary basin opening has been demonstrated for the South American Andes and one of the few, known cases worldwide.
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Testing the accuracy of OSL and pIR IRSL dating of young geoarchaeological sediments in coastal Peru
(Elsevier B.V., 2022-11-01)
Luminescence dating of late Quaternary sediments in Peru is challenging, especially on the Peruvian coast. Earlier studies have shown that quartz grains often exhibit a thermally unstable, medium signal that caused the underestimation of Optically Stimulated Luminescence (OSL) ages. InfraRed Stimulated Luminescence (IRSL) dating has shown to produce more reliable ages, depending, amongst other factors, on the age model (Central or Minimum Age Model), and the IRSL signal. IRSL dating of geoarchaeological sediments has, however, hardly been carried out, let along validated, against an independent age dataset. This dating approach is, nonetheless, the only promising way to date the geological substrate in which many of Peru's archaeological sites are buried. Peru contains some of the oldest and most important archaeological heritage sites, yet not much is known of the environmental context in which many of its early civilizations prospered. A better understanding of which luminescence method works best could therefore help in a better understanding of the geological-stratigraphical context of many of Peru's sites. To investigate this matter more fully, we compared the luminescence dating results of seven sediment samples from the top layer of the Lima alluvial fan and from geoarchaeological layers of the Maranga Complex (San Miguel, Lima), with an independent dataset of sixteen 14C ages. Our results showed that the quartz OSL ages always underestimated the expected ages due to a signal dominated by medium and slow components, and that the post-IR IRSL225 (pIR IRSL225) and IRSL50 ages of K-feldspars, on basis of the Central Age Model (CAM), always overestimated the expected ages. The Minimum Age Model (MAM) on the other hand, correctly predicted the expected ages for the early Holocene, Lima alluvial fan sediments using the pIR IRSL225 signal of K-feldspars, and the late Holocene, geoarchaeological ages using the IRSL50 signal.
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Heinrich events and tectonic uplift as possible drivers for late Quaternary fluvial dynamics in the western Peruvian Andes
(Elsevier B.V., 2022-11-01)
Late Quaternary fluvial valley development in Peru is considered to be driven by summer insolation maxima in the precession cycle, which coincide with increased precipitation and rising lake levels in the central Andes. Our literature review, however, indicates that fluvial aggradation does not always coincide with summer insolation maxima but may also occur during transitions between insolation maxima and minima or even during summer insolation minima. Tectonic uplift as a driver of fluvial incision has not been considered in the current terrace formation models, despite the Peruvian margin being located above an active subduction zone. We present new chronologic, stratigraphic and geomorphic data of the Cañete River valley over the past 102 ± 6 ka. We mapped its fluvial terraces (11.5°-13° southern latitude) along a 75-km long reach perpendicular to the strike of the Andes. Five fluvial terraces and one floodplain level were identified with relative elevations of up to 181.1 m above the floodplain. Thirty-three (pIR) IRSL ages of eight fluvial terraces and one alluvial fan sediment sample showed that their ages in part correspond to summer insolation maxima of the precession cycle, but that a better match exists with pluvial periods that coincided with Heinrich events of the northern hemisphere and rising lake levels in the Andes. The chronology of terraces of the Cañete River agrees with those of previously studied fluvial systems between 7° and 16° southern latitude in Peru, suggesting a regional-scale fluvial response possibly to the Heinrich events. Reconstructed longitudinal profiles and terrace ages were utilised to calculate vertical incision rates. We calculated a mean rate of 1.8 ± 0.10 mm a−1 over the last 102 ± 6 ka, but incision rates varied considerably within this time period possibly in response to changes in the Qs: Qw ratio as a consequence of increased sediment input during the wet Heinrich events. A set of 1338 GNSS measurements of daily, vertical crustal deformation of the years 2009–2015 showed a continuous and positive trend in interseismic, accumulative vertical crustal movements with a time-averaged, crustal uplift rate of 1.9 ± 3.6 mm a−1. The positive trend in vertical movements, together with data from literature suggest landscape rejuvenation along the forearc and western Cordillera in central Peru. It is proposed that maximally ∼0.5 mm a−1 of the total incision over the past 102 ka may possibly be related to nonrecoverable, interseismic deformation. Climate change-driven variations in the Qs:Qw ratio are superimposed on the long-term tectonic uplift trend, and both are considered the main drivers of fluvial incision.
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Spatial variation in specific sediment yield along the Peruvian western Andes
(Elsevier BV, 2022-10-22)
The tropical Andes has been less studied in terms of erosion processes in comparison to other major mountain ranges in the world. Environmental gradients are steepest along the western flank of the Andes that is characterized by marked differences in vegetation, rough topography with deeply incised canyons, and highly variable and extreme precipitation patterns. Previous efforts mostly focused on sediment fluxes in large rivers draining to e.g. the Amazon basin while small to medium-sized rivers such as the ones flowing towards the Pacific Ocean have been relegated. They highlighted the link between sediment yield, anthropogenic and natural factors, e.g. climate, topography, river runoff, lithology and vegetation cover. In this study, we identified the spatial patterns of specific sediment yield along the western slopes of the Peruvian Andes between 3° and 13° S latitude for 21 catchments. We collected and analysed data from 22 environmental factors to elucidate their importance on spatially varying sediment yield. The sediment load was derived from gauging stations, reservoir sedimentation and water turbidity over a 30-yr period. The specific sediment yield varies strongly along the Peruvian western Andes as a consequence of the spatial variation in climate, topography and land cover controlling sediment production and transport. We reported higher-than-average specific sediment yields for the central part (6°-11°S) with values of 2130 and 2300 t km−2 yr−1 and low and uniform yields of 39 to 551 t km−2 yr−1 in the southern part (11° − 14.5° S). Given the scarcity of data on sediment yield, we included an uncertainty assessment based on bootstrapping approaches as to get a better grasp on the potential range of specific sediment yields in the study region. Using statistical techniques including Spearman correlation rank, univariate and multivariate regression analyses, we were able to determine the importance of the 22 environmental variables on the specific sediment yield. About 55 % of the observed variance can be explained by river discharge (Q90) and river steepness index (ks50). By adding an anthropogenic variable based on land cover, the explained variance in SSY increases up to 63 %, however, the effects of land cover on specific sediment yield are not clear because of spurious correlation between land cover, river discharge and topography. Our study therefore provides important new insights in the ongoing scientific debate on sediment yield variability in the western Andes.

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