Characterization and Distribution of Pyrogenic Carbon in a Fraction of Archaeological Black Earth from Caxiuanã

Moraes, Milena Carvalho; Lemos, Vanda Porpino; Moraes, Dorsan Santos; Moraes, Cláudio Nery Lamarão

doi:10.5935/0103-5053.20150140

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Journal of the Brazilian Chemical Society

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Articles • J. Braz. Chem. Soc. 26 (8) • Aug 2015 • https://doi.org/10.5935/0103-5053.20150140 copy

Characterization and Distribution of Pyrogenic Carbon in a Fraction of Archaeological Black Earth from Caxiuanã

Authorship SCIMAGO INSTITUTIONS RANKINGS

This study aims to determine the atomic ratio of O/C in an archaeological black earth (ABE) profile of the Ilha de Terra site, a region of Caxiuanã in the Pará State, Brazil, to determine the types of pyrogenic carbon (PyC) particles and to infer the source of biomass and burning temperature necessary to produce the PyC. The O/C ratios were monitored using scanning electron microscopy combined with energy dispersive spectroscopy (SEM/EDS). The results indicated atomic ratios for clay, silt and fine sand fractions that were between those registered for the PyC particles types: condensed combustion (CC) (0.09, 0.1, 0.13), charcoal (0.32, 0.31, 0.34) and char (0.43, 0.45, 0.52). CC is the predominant type of particle found because of the high firing temperature (> 350 °C), which is consistent with the probable biomass sources of wood, cellulose and lignin.

Keywords
pyrogenic carbon; black earth; O/C atomic ratio; condensed combustion

Horizon	Thickness / cm	Color	Code color	Texture	Clay	Silt	Fine sand	Coarse sand
A1	7	black	10YR2/1	sandy	(A1)/1	(A1)/29	(A1)/15	(A1)/43
A2	7	black	10YR2/1	sandy	(A2)/2	(A2)/30	(A2)/16	(A2)/44
A3	15	dark brown	7,5YR3/2	sandy	(A3)/3	(A3)/31	(A3)/17	(A3)/45
AB	28	dark brown	7,5YR4/6	sandy	(AB)/4	(AB)/32	(AB)/18	(AB)/46
BA	32	brown	7,5YR4/3	sandy	(BA)/5	(BA)/33	(BA)/19	(BA)/47
B1	21	yellowish brown	10YR5/4	sandy-clay	(B1)/6	(B1)/34	(B1)/20	(B1)/48
B2	25	yellowish brown	10YR5/4	sandy-clay	(B2)/7	(B2)/35	(B2)/21	(B2)/49

Step	Method	Reference	Step	Method	Reference
(1) Distinction of carbonaceous particles	scanning electron microscopy (SEM)	13,17,34,36	(4) rejection of O/C values	Dixon test	17,34
(2) Monitoring of O/C atomic ratio	energy dispersive spectroscopy (EDS)	17,34	(5) classification of PyC particles	values of O/C: > 0 and < 0.6	17,34
(3) Correction of matrix effects	position of the electron beam	17,34	(6) evaluation of precision and accuracy	based on standard deviation (SD) and relative error values	45

Element	a.1 / wt.%	a.2 / wt.%	a.3 / wt.%	a.6 / wt.%	b.1 / wt.%	b.2 / wt.%	b.3 / wt.%	b.4 / wt.%	b.5 / wt.%
C	88.64	88.67	87.46	87.81	88.56	93.02	88.13	92.2	86.4
O	10.07	10.05	11.5	11.01	10.42	6.04	10.96	5.98	12.88
Na	0.07	0.08	0.08	0.05	0.05	0.06	0.05	0.03	0.05
Mg	0.003	0.01	0.02	0.007	0.03	0.03	0.003	0.03	0.01
Al	0.038	0.007	0.01	0.06	not detected	not detected	0.011	0.09	0.001
Si	0.06	0.007	0.007	0.004	0.04	0.09	0.006	0.05	0.019
P	0.93	0.87	0.72	0.74	0.61	0.53	0.66	1.35	0.51
K	0.05	0.06	0.03	0.06	0.08	0.09	0.016	0.04	0.41
Ca	0.057	0.06	0.06	0.12	0.08	0.04	0.05	0.12	0.024
Fe	0.072	0.09	0.09	0.12	0.12	0.09	0.11	0.08	0.06
O/C	0.082	0.084	0.107	0.099	0.096	0.046	0.103	0.023	0.132

Property	Reference material	Average value	n	Standard deviation	Variance	Relative error / %
O/C	activated carbon Merck	0.0586	4	0.03	0.0009	0.17
O/C	activated carbon Merck	0.1074	5	0.014	0.0002	0.024
O/C	commercial charcoal¹⁷17 Stoffyn-Egli, P.; Potter, T. M.; Leonard, J. D.; Pocklington, R.; Sci. Total Environ. 1997, 198, 211.	0.05	7	0.01	0,0001	-
O/C	coal from soft wood combustion³⁴34 Brodowski, S.; Amelung, W.; Haumaier, L.; Abetz, C.; Zech, W.; Geoderma 2005, 128, 116.	0.11	8	0.02	0.0004	-

Element	(A/1)1 / wt.%	(A/1)2 / wt.%	(A/1)3 / wt.%	(A/1)5 / wt.%	(A/1)6 / wt.%	(A/1)8 / wt.%
C	58.45	53.96	54.48	55.55	59.83	48.47
O	32.89	31.2	30.2	34.06	25.21	33.13
Na	0.06	0.08	0.2	0.06	0.17	0.1
Mg	0.56	0.66	0.78	0.42	0.66	0.2
Al	0.7	0.63	0.64	0.76	0.56	4.89
Si	1.24	2.34	1.41	3.79	1.41	7.38
P	0.61	1.22	1.1	0.91	1.02	0.99
K	0.083	0.16	1.15	0.1	0.07	0.26
Ca	5.07	9.42	10.69	3.95	10.8	1.31
Fe	0.33	0.32	0.35	0.36	0.26	3.26
O/C	0.37	0.22	0.21	0.32	0.12	0.06

Element	A2/2a1	A2/2a2	A2/2a5	A2/2b1	A2/2b2	A2/2c1	A2/2c2	A3/3-1	AB/4b5	AB/b6
C	59.5	63.3	59.6	52.07	51.55	57.18	46.76	48.65	43.1	46.4
O	34.53	31.3	35.4	38.27	36.03	34.47	35.44	35.5	17.9	28.1
Na	0.05	0.02	0.15	0.1	0.08	0.07	0.03	0.2	0.1	0.2
Mg	0.18	0.16	0.32	0.33	0.42	0.29	0.16	0.13	0.1	0.03
Al	1.38	1.6	0.65	2.25	2.73	0.67	1.49	5.6	6.6	1.7
Si	1.66	1.76	2.66	3.92	5.38	4.67	11.12	6.85	22.2	16.1
P	1.32	0.7	0.08	0.23	0.64	0.62	1.1	0.798	2.6	0.6
K	0.04	0.04	0.03	0.27	0.11	0.05	0.09	0.13	0.7	0.4
Ca	0.84	0.69	1.28	1.76	1.87	1.73	1.25	0.61	1.1	0.2
Fe	0.51	0.49	0.22	0.77	0.89	0.24	2.56	0.82	5.6	6.2
O/C	0.4063	0.3494	0.4466	0.4471	0.3183	0.3535	0.0803	0.2004	-1.18	-0.35

Element	(A1)29-7 / %	(A1)29-22 / %	(A2) 30-4 / %	(A3) 31-19 / %	(A3)31-22 / %	(AB)32-1 / %	(AB)32-2 / %
C	57.97	64.22	71.31	65.4	63.1	60.54	59.67
O	34.85	29.26	19.59	28.96	28.26	31.49	27.35
Na	0.2	0.16	0.33	0.34	0.38	0.32	0.3
Mg	0.36	0.01	0.1	0.01	0.09	0.03	0.04
Al	0.48	0.1	0.58	0.41	1.8	0.47	2.23
Si	1.86	5.13	6.52	4.46	5.34	5.03	9.28
P	1	0.56	0.74	0.11	0.16	0.13	0.1
K	0.04	0.05	0.12	0.04	0.04	0.05	0.1
Ca	2.76	0.26	0.5	0.14	0.2	0.23	0.2
Fe	0.96	0.22	0.23	0.13	0.62	0.69	0.73
O/C	0.3984	0.2609	0.0388	0.2852	0.2098	0.2912	0.0655

Element	15/A1/1 / %	15/A1/2 / %	15/A1/3 / %	16/A2/2 / %	16/A2/4 / %	17/A3/2 / %	17/A3/7 / %	18/AB-1 / %
C	63.1	51.34	50.11	48.27	50.47	46.5	46.47	54.91
O	26.76	31.3	37.83	29.08	31.8	39.74	39.99	35.41
Na	0.05	0.02	0.02	0.15	0.1	0.06	0.056	0.16
Mg	0.4	0.39	0.15	0.9	0.45	0.16	0.29	0.07
Al	0.24	3	0.75	1.06	5.47	3.23	3.59	0.63
Si	2.27	3.65	8.1	2.7	4.13	7.78	6.02	6.27
P	1.47	2.38	0.86	0.97	1.34	0.72	0.76	0.24
K	0.09	0.28	0.07	0.16	0.29	0.12	0.15	0.11
Ca	5.42	5.82	1.63	15.95	4.1	0.83	1.72	0.08
Fe	0.19	1.86	0.5	0.75	1.85	0.85	0.94	2.13
O/C	0.1913	0.0973	0.3188	0.03671	0.1007	0.3322	0.3551	0.3377

Horizon	Clay	Silt	Fine sand	Coarse sand	Total of particles
A1	19	11	26	1	57
A2	23	4	8	0	35
A3	8	7	2	0	17
AB	0	4	1	0	5

Description	Clay	Silt	Fine sand	Coarse sand
Total char particles	5	1	8	1
Total charcoal particles	25	11	16	0
Total condensates of combustion particles	20	24	13	0

Fraction	O/C	SD	n	Type of particles
Clay	0.09	0.07	21	condensed combustion
	0.32	0.05	17	charcoal
	0.43	0.01	8	char
Silt	0.1	0.05	15	condensed combustion
	0.31	0.07	11	charcoal
	0.45	-	1	char
Fine sand	0.13	0.07	15	condensed combustion
	0.34	0.05	14	charcoal
	0.52	0.04	8	char

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