Energy regime of the geodynamo during the Cretaceous Normal Superchron via paleosecular variation and paleointensity

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.