D. Khider

A new perspective on the hydroclimate variability in northern South America during the Little Ice Age

[1] An absolute dated speleothem oxygen isotope (δ 18 O) record from northeastern Peru documents monsoon precipitation variability over northern South America during the past that 1000 years and indicates the annual precipitation in the 15th through the 18th centuries, the so-called Little Ice Age (LIA), was on average ∼10% higher than during the 20th century. Over the 20th century recurrent modes of seasonal rainfall variability across northern South America were associated with discrete sea surface temperature anomaly patterns within the Atlantic and Pacific Oceans. Calling upon these SST-rainfall teleconnectivity patterns, and paleo-SST reconstructions that span the past 8 centuries, higher annual rainfall across northern South America during the LIA is attributed to cooler boreal spring SSTs in the tropical North Atlantic. Weaker co-variance between north Atlantic SSTs and the South American Monsoon System (SAMS) rainfall during the 20th century suggests that ENSO has become a more dominant influence than it was during the LIA.

Assessing El Niño Southern Oscillation variability during the past millennium

evaluate the relative strength/frequency of El Nino and La Nina events. In contrast to previous studies, we use robust and resistant statistics to quantify the spread and symmetry of the d 18 O distributions; an approach motivated by the relatively small sample size and the presence of outliers. Furthermore, we use a pseudo‐proxy approach to investigate the effects of the different paleo‐environmental factors on the statistics of the d 18 O distributions, which could bias the paleo‐ENSO reconstruction. We find no systematic difference in the magnitude/strength of ENSO during the Northern Hemisphere MCA or LIA. However, our results suggest that ENSO during the MCA was skewed toward stronger/more frequent La Nina than El Nino, an observation consistent with the medieval megadroughts documented from sites in western North America.

A Controlled Crowdsourcing Approach for Practical Ontology Extensions and Metadata Annotations

Traditional approaches to ontology development have a large lapse between the time when a user using the ontology has found a need to extend it and the time when it does get extended. For scientists, this delay can be weeks or months and can be a significant barrier for adoption. We present a new approach to ontology development and data annotation enabling users to add new metadata properties on the fly as they describe their datasets, creating terms that can be immediately adopted by others and eventually become standardized. This approach combines a traditional, consensus-based approach to ontology development, and a crowdsourced approach where expert users (the crowd) can dynamically add terms as needed to support their work. We have implemented this approach as a socio-technical system that includes: (1) a crowdsourcing platform to support metadata annotation and addition of new terms, (2) a range of social editorial processes to make standardization decisions for those new terms, and (3) a framework for ontology revision and updates to the metadata created with the previous version of the ontology. We present a prototype implementation for the Paleoclimate community, the Linked Earth Framework, currently containing 700 datasets and engaging over 50 active contributors. Users exploit the platform to do science while extending the metadata vocabulary, thereby producing useful and practical metadata.

The role of uncertainty in estimating lead/lag relationships in marine sedimentary archives: A case study from the tropical Pacific†

Understanding the mechanisms behind any changes in the climate system often requires establishing the timing of events imprinted on the geological record. However, these proxy records are prone to large uncertainties, which may preclude meaningful conclusions about the relative timing of events. In this study, we put forth a framework to estimate the uncertainty in phase relationships inferred from marine sedimentary records. The novelty of our method lies in the accounting of the various sources of uncertainty inherent to paleoclimate reconstruction and timing analysis. Specifically, we use a Monte-Carlo process allowing sampling of possible realizations of the time series as functions of uncertainties in time, the climate proxy, and the identification of the termination timing. We then apply this technique to 15 published sea surface temperature records from the equatorial Pacific to evaluate whether we observed any significant changes in the termination timing between the East and the West. We find that the uncertainty on the relative timing estimates is on the order of several thousand years, and mainly stems from age model uncertainty (90%). However, even small differences in mean termination timings can be detected with a sufficiently large number of samples. Improvements in the dating of sediment records provide an opportunity to reduce uncertainty in studies of this kind.

Decadal to centennial fluctuations in the intensity of the eastern tropical North Pacific oxygen minimum zone during the last 1200 years

Oxygen minimum zones (OMZs), located below highly productive marine regions, are sites of microbially mediated denitrification and biogeochemical cycling that have global significance. The intensity of OMZs fluctuates naturally; however, the degree of these fluctuations and a comprehensive understanding of the factors that drive these fluctuations on decadal to centennial time scales is lacking. Our high-resolution (near-annual) record of δ15Nsed from laminated sediments at the Pescadero Slope in the Gulf of California (eastern tropical North Pacific) fluctuates between maximum values of 10.5‰ and minimum values of 8.0‰ over the past 1200 years. An analysis of the relationship between δ15NO3− and [O2] in the water column suggests that the observed range of δ15Nsed values is equivalent to an approximately 8 µM fluctuation in O2 content and that these changes can occur in less than 25 years. Our findings show that the OMZ typically intensifies quickly and contracts gradually; the average rate of OMZ intensification (−0.24 µM O2/yr) is twice as fast as the rate of OMZ reoxygenation. Spectral analyses of the δ15Nsed record and Br/Cl counts, with the latter are used as a proxy for organic carbon preservation, suggest that the Pacific Decadal Oscillation and the Suess (deVries) solar cycle (solar irradiance) may influence the intensity of the OMZ and carbon production/export during the late Holocene. Coherence between δ15Nsed and weight percent organic carbon also suggests that similar mechanisms influence both OMZ fluctuations and variation in organic carbon production/export.