5.0 Discussion
In this study, geophysics played a significant role in recognising the
Bosumtwi as a terrestrial impact crater. Twenty-three (23) geophysical
papers on the Bosumtwi impact crater were identified from various
databases, and 47.8% of these publications were published in 2006 and
2007 attributable to the multidisciplinary International Continental
Drilling Program (ICDP), a project that took place in 2004. We also
found that 13% of these published scientific articles on the Bosumtwi
Impact Crater were written by researchers affiliated with Ghanaian
institutions, whereas 87% of the studies were conducted by scientists
affiliated with non-Ghanaian institutions. Our findings are significant
because they highlight the lack of local financial support for research
in Ghana and African countries. The lack of funding for research on the
Bosumtwi Impact Crater from Ghana also bears this out. One would have
thought that with such a unique location located in the country,
adequate funding would have been allocated for research to advance
knowledge.
In addition to our main findings, we demonstrate that the geophysical
techniques employed so far in studying the Bosumtwi Impact crater
include magnetic, gravity, seismic, electrical resistivity, GPR,
petrophysical, remote sensing and radiometric methods. A residual
negative gravity anomaly is the most conspicuous geophysical signature
over impact craters. Gravity measurements at the Bosumtwi impact crater
yielded a maximum negative anomaly of 18 mgal over the crater. This was
interpreted to be caused by fractured and brecciated rocks in the rim
area and below the crater floor, breccias within the crater, and
sedimentary and water infilling of the lake. Also, magnetic measurements
of core samples, borehole measurements, and magnetic measurements over
the crater reveal interesting anomalies. A magnetic low with an
amplitude of tens to several hundred nanotesla (nT) is the dominant
effect over impact. A large negative anomaly with a minimum value of 55
nT and a less notable positive anomaly to the south of the Bosumtwi
impact crater was recorded.
In contrast, weaker negative anomalies were also observed in the
southeast and southwest of the crater, which is attributed to magnetised
bodies in the central northern area of the lake. Measurements of
magnetic susceptibility of the drill cores show mostly paramagnetic
values (200–500 × 10–6 SI) throughout the core,
except for a few metasediment samples, and correlate positively with
natural remanent magnetisation (NRM) and Q values. Seismic refraction
and reflection surveys showed the velocity distribution in the Bosumtwi
Impact Crater, which expressed how much the shock load has fractured the
material. The methods revealed a three-layer model consisting of the
water layer with a velocity of 1.45 km/s and a higher velocity of
between 1.5 km/s to 1.65 km/s, interpreted as post-impact sediments and
the crater floor. Resistivity measurement at the crater reveals three
formations with distinct resistivity signatures: low resistivity regions
from the lake’s shore to uphill with resistivities of 64 Ω.m
representing lake sediments; moderately high resistivity regions with
values between 128 and 200 Ω.m interpreted as impact-related breccias
such as dikes, allochthonous or parautochthonous depending on their
geometries; and a resistive basement metamorphic formations.
The resistivity models were able
to determine faults and fractures, as well as the thickness of
post-impact lake sediments and breccias.
It is clear from our study that geophysical studies in the Bosumtwi
Impact crater have stalled after a high in 2006 and 2007 (i.e., the
period immediately after the Lake Bosumtwi Drilling Project in 2004. The
Lake Bosumtwi Drilling Project was funded by a consortium of the
International Continental Scientific Drilling Program (ICDP), the US
National Science Foundation, the Austrian National Science Foundation,
the Austrian Academy of Sciences
and the Canadian National Science Foundation. Local funding from the
Government of Ghana seems to be nonexistent. This study reinforces the
overwhelming evidence of chronic internal underfunding of science
research in Ghana and Africa (North et al., 2020; Sawyerr, 2004 ).North et al. (2020 ) also identify government spending on
research and development as a key reason for low research output from
African countries such as Ghana. Over the years, the Government of Ghana
has not dedicated resources to investigating the origin, understanding
the evolution and encouraging the
continuous monitoring of the Bosumtwi Impact Crater. An unintended
consequence is that data from geophysical research conducted on the
Bosumtwi Impact Crater is unavailable to Ghanaian researchers. For
instance, the data gathered during the Lake Bosumtwi Drilling Project
should have been in an accessible research repository to Ghanaian
researchers, but this is not the case. Original plans to have a museum
and research centre at Abonu, the main town in the Bosumtwi Impact
Crater enclave, to host some of the petrophysical cores, rocks and
results from research conducted have not materialised almost 20 years
after the ICDP ended. Although Ghanaian researchers are underrepresented
in geophysical research on the Bosumtwi Impact Crater, we do not ascribe
the cause to “parachute science” (Harris, 2004; North et al.,
2020 ), as evidenced by 19 % of the scientists who worked on Bosumtwi
Impact Crater being Ghanaian (Koeberl et al., 2005 ). If anything,
the internal evils of lack of resources for research and publications,
high teaching loads, and lack of institutional incentives are to blame
for the under-representation of African geophysical researchers in
studies in the Bosumtwi Impact Crater.
Another significant aspect of our review is the importance of geophysics
in understanding impact craters. The geophysical techniques of magnetic,
gravity, seismic, electrical resistivity, GPR, petrophysical, remote
sensing and radiometric methods were all utilised in the various
investigations of the Bosumtwi Impact Crater. Meteorite Impact Craters
are pervasive in our solar system, and studying them is useful for
understanding the geological processes of other planetary bodies in our
solar system, such as Mars, especially in the context of ramped-up
exploration of other planetary bodies in the solar system. They are
often the most significant modifiers of planetary surfaces. However, our
review makes it clear that while geophysical methods did not provide an
unambiguous signature for the Bosumtwi Impact crater, they provided
constraints for confirming the impact origins. Our review supports
French and Koeberl’s (2010) and Koeberl’s (2004) findings.
In our view, a very compelling consequence of the underrepresentation of
Ghanaian geophysical researchers in studies on the Bosumtwi Impact
Crater is the lack of studies on hazards due to the impact origins of
the impact crater. For instance, Jones (1985 ) reported that the
earliest mention of Lake Bosumtwi was by a certain Perregaux in
1899, who described inhabitants of the Lake Bosumtwi enclave
complaining of bubbles accompanied by sulphurous smell and loud noises
in a two-year cycle. Scientists then concluded that this occurrence,
along with the crater depression, indicated volcanic origins. Since this
report by Jones (1985 ), we have not found any study investigating
this phenomenon. This record should be of particular concern to
scientists conducting future research since it could have health and
safety implications for the inhabitants of the Lake Bosumtwi enclave.
Furthermore, beyond geophysical research for understanding the origin
and evolution of the Bosumtwi Impact crater, future research should
focus on geological hazards related to impact, which may put the lives
and properties of inhabitants of the enclave at risk. In addition, the
potential for contamination of the lake through fractured conduits
should be investigated. The Bosumtwi enclave is a zone of high
agricultural activities, especially cocoa and other crops such as maise,
cassava and plantain. Some farmers involved in these agricultural
activities use high amounts of
pesticides and inorganic fertilisers. The potential for contamination of
the lake due to seepage through the fractures is high. Finally, the
rapid increase in tourist numbers visiting the lake and the construction
of several tourist resorts in the enclave should engender geotechnical
research using geophysical methods in the future.