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Kollofrath, Julia (2012): Energy regime of the geodynamo during the Cretaceous Normal Superchron via paleosecular variation and paleointensity. Dissertation, LMU München: Faculty of Geosciences
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Abstract

The Earth's magnetic field underwent hundreds of reversals during its history. But within a ~40 Myr span (84-125 Ma) during the Cretaceous no reversal happened. For comparison, the second longest chron length during the last 167 Ma is ~5 Myr. Thus, the ~40 Myr long chron is known as a superchron and is called Cretaceous Normal Superchron (CNS). Two other superchrons are now established: the Permian-Carboniferous Reversed Superchron and the Ordovician Reversed Superchron. Why do these superchrons exist? Are they an extreme chron duration of the same statistical distribution? Or, do superchrons reflect a distinct dynamo regime separate from an oft-reversing regime. Are the onset and end of superchrons triggered by changes in the physical conditions of outer core convection? For example, instabilities within the convection in the outer core are suspected to trigger reversals. A `low energy' geodynamo during the superchron could stem from less turbulent convection. But also the concept of a `high energy' geodynamo during a superchron is conceivable: stronger convection would stabilize the field and increase the field intensity. These different dynamo regimes could be be triggered by changing the temperature conditions at the core mantle boundary (CMB), for example with the eruption of deep mantle plumes or the descent of cold material such as subducted slabs. Insights into past geodynamo regimes can be learned primarily from two paleomagnetic methods: paleosecular variation (variation in field directions) and paleointensity. For the former, we collected 534 samples for a paleosecular variation study from a 1400 m-long, paleontologically well-described section in northern Peru. Thermal demagnetization isolates stable magnetization directions carried by greigite. Arguments are equivocal whether this remanence is syn-diagenetic, acquired during the Cretaceous normal superchron, or a secondary overprint, acquired during a chron of solely normal polarity in the upper Cenozoic, yet pre-Bruhnes (>800 kyr). We explore the ramifications on the S value, which quantifies paleosecular variation, that arises from directional analysis, sun compass correction, bedding correction, sampling frequency, outlying directions and different recording media. The sum of these affects can readily raise the S value by more than 20%. S values from northern Peru are indistinguishable from other S values for the Cretaceous normal superchron as well as those for the last 5 Ma. Summing over all the potential uncertainties, we come to the pessimistic conclusion that the S value is an unsuitable parameter to constrain geodynamo models. Alternatively, no statistical difference in paleosecular variation exists during much of the Cretaceous normal superchron and during the last 5 Ma. Even though the S value might be unsuitable, we wanted to understand why the S value is latitude dependent. The origin of this latitude dependency is widely attributed to a combination of time-varying dipole and non-dipole components. The slope and magnitude of S are taken as a basis to understand the geomagnetic field and its evolution. Here we show that S stems from a mathematical aberration of the conversion from directions to poles, hence directional populations better quantify local estimates of paleosecular variation. Of the various options, k is likely the best choice, and the uncertainty on k(N) was already worked out. As we came to the pessimistic conclusion that the S value might not be the best parameter to quantify the `energy state' of the geodynamo during a superchron, we also carried out a paleointensity study on 128 samples from volcanic rocks in Northern Peru and Ecuador. Oxidation of the remanence carriers was a problem. Only one site gave reliable results. Two methods of paleointensity determination were applied to these rocks. The results of both methods agree quite well with each other and also with previous studies from other sites. Our results suggest that the field intensity towards the end of the superchron seems to quite similar to today's magnetic moment. Thus, it can be concluded that the `energy state' of the geodynamo was not substantially different during the Cretaceous Normal Superchron compared to reversing times. Why do superchrons exist? One possible explanation is that paleomagnetism is not able to resolve different energy states of the geodynamo, neither with paleosecular variation nor with paleointensity. This was suggested by some dynamo simulations in which the heat flux across the core-mantle boundary was kept the same, but the resulting paleosecular variation, paleointensity and frequency of reversals differed a lot. Another possible explanation is that a superchron is an intrinsic feature of the distribution of magnetic polarity chron lengths. Thus, no changes of the convection in the outer core are needed to trigger a superchron.