The global paleointensity database for 65-200 Ma was analyzed using a modified suite of paleointensity quality criteria (Q PI ) such that the likely reliability of measurements is assessed objectively and as consistently as possible across the diverse data set. This interval was chosen because of dramatic extremes of geomagnetic polarity reversal frequency ranging from greater than 10 reversals per million years in the Jurassic hyperactivity period (155-171 Ma) to effectively zero during the Cretaceous Normal Superchron (CNS; 84-126 Ma). Various attempts to establish a relationship between the strength of Earth's magnetic field and the reversal frequency have been made by previous studies, but no consensus has yet been reached primarily because of large uncertainties in paleointensity estimates and sensitivity of these estimates to data selection approaches. It is critical to overcome this problem because the evolution of the dipole moment is a first order constraint on the behavior of the geodynamo. Here we show that conventional statistical tests and Bayesian changepoint modeling consistently indicate the strongest median/average virtual dipole moment during the CNS. In addition, the CNS and Jurassic hyperactivity period are characterized by the highest and lowest percentage of virtual dipole moments exceeding the overall median for the 65-to 200-Ma interval, respectively. These observations suggest that the superchron dynamo was able to generate stronger fields than the dynamo operating in the frequently reversing regime. While the precise mechanism remains unclear, our results are compatible with the hypothesis that field strength and reversal rate variation are controlled by changes in core-mantle boundary thermochemical conditions.
Summary Palaeomagnetic field intensity measurements, derived from rocks with ages that span geological time, provide a crucial constraint on the evolution of Earth’s deep interior and its magnetic environment. The palaeointensity database PINT has been updated to version v.8.0.0 and includes palaeointensity site-mean records spanning an interval from 50 ka to 4.2 Ga, compiling efforts from the palaeomagnetic community spanning from 1959 to the end of 2019. Nearly all site-mean palaeointensity records have been assessed using the qualitative reliability of palaeointensity (Quality of Palaeointensity, QPI) framework. This updated database brings together and harmonizes prior QPI and PINT compilation efforts into a unified database referred to as the PINT database, incorporating recent efforts since 2014 to assess QPI. The spatio-temporal distribution of the PINT database is analyzed, revealing substantial biases towards young records (from the Brunhes chron) in the Northern hemisphere, and intervals with little to no palaeointensity data with a duration of 10s to 100s of millions of years in the Paleozoic and Precambrian. General QPI compliance is characterized for the PINT database, which shows that the median QPI scores range from 2 to 3 (out of a total possible score of 10), with a positive trend towards increasing QPI scores in studies published after the year 2000. This illustrates an increasing community awareness of what is required to establish confidence in palaeointensity data and an increasing robustness of the large scale interpretations that can be made with these data. We additionally present a description of the long-term average dipole field strength with descriptive statistics for distinct intervals of Earth history.
PurposeCollaborative knowledge building (CKB) is seen as a means for achieving desired learning outcomes as well as facilitating sharing and distribution of knowledge among community members. However existing CKB studies do not appear to identify and account for the tools used by groups (at individual and group level) as part of the CKB process. The paper aims to address this issue.Design/methodology/approachThe paper describes a group knowledge building exercise within an educational context using activity theory as a descriptive data analysis tool. Data analysis involved conceptualising the CKB process as an activity system in which the group worked towards a shared object and identifying the ensuing contradictions in the CKB activity system.FindingsResults from the analysis illustrate participants' use of reflective thinking processes for resolving contradictions and as a tool for articulating knowledge and developing a shared understanding. Two types of contradictions are identified from the analysis resolving which helped the group to achieve their objective. The efficacy of using activity systems as a holistic and flexible unit of analysis for studying CKB is illustrated through discussion of the results.Research limitations/implicationsThe results have educational research implications in terms of developing research tools for analysing CKB, collecting data from a group context, and developing tools for improving group‐work.Practical implicationsThe results have practical implications in terms of building knowledge from experience within knowledge communities.Originality/valueOne of the outcomes of the study is the identification of developmental and reflective contradictions which highlight the issues that when addressed allow for successful achievement of the object (or to some degree of success), as well as a richer deeper experience for the participants.
This paper describes a model for studying collaborative knowledge building (CKB) as a group activity. We integrate the model described by Stahl (2000a) with an analysis based on the principles of cultural historical activity theory (CHAT), using the analysis of a self reflective case study to guide the description. The concept of the CKB activity system is developed, and the role of contradictions in CHAT is described. The case is then analyzed to show how the model explains collaboration in practice. The final model includes two additional cycles representing the role of reflective practice in CKB. The new model of CKB processes combined with the concept of the activity as the unit of analysis and the tools of CHAT provides an efficacious way of investigating collaborative knowledge building.
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